# When Greedy Algorithms are Perfect: the Matroid

Greedy algorithms are by far one of the easiest and most well-understood algorithmic techniques. There is a wealth of variations, but at its core the greedy algorithm optimizes something using the natural rule, “pick what looks best” at any step. So a greedy routing algorithm would say to a routing problem: “You want to visit all these locations with minimum travel time? Let’s start by going to the closest one. And from there to the next closest one. And so on.”

Because greedy algorithms are so simple, researchers have naturally made a big effort to understand their performance. Under what conditions will they actually solve the problem we’re trying to solve, or at least get close? In a previous post we gave some easy-to-state conditions under which greedy gives a good approximation, but the obvious question remains: can we characterize when greedy algorithms give an optimal solution to a problem?

The answer is yes, and the framework that enables us to do this is called a matroid. That is, if we can phrase the problem we’re trying to solve as a matroid, then the greedy algorithm is guaranteed to be optimal. Let’s start with an example when greedy is provably optimal: the minimum spanning tree problem. Throughout the article we’ll assume the reader is familiar with the very basics of linear algebra and graph theory (though we’ll remind ourselves what a minimum spanning tree is shortly). For a refresher, this blog has primers on both subjects. But first, some history.

## History

Matroids were first introduced by Hassler Whitney in 1935, and independently discovered a little later by B.L. van der Waerden (a big name in combinatorics). They were both interested in devising a general description of “independence,” the properties of which are strikingly similar when specified in linear algebra and graph theory. Since then the study of matroids has blossomed into a large and beautiful theory, one part of which is the characterization of the greedy algorithm: greedy is optimal on a problem if and only if the problem can be represented as a matroid. Mathematicians have also characterized which matroids can be modeled as spanning trees of graphs (we will see this momentarily). As such, matroids have become a standard topic in the theory and practice of algorithms.

## Minimum Spanning Trees

It is often natural in an undirected graph $G = (V,E)$ to find a connected subset of edges that touch every vertex. As an example, if you’re working on a power network you might want to identify a “backbone” of the network so that you can use the backbone to cheaply travel from any node to any other node. Similarly, in a routing network (like the internet) it costs a lot of money to lay down cable, it’s in the interest of the internet service providers to design analogous backbones into their infrastructure.

A minimal subset of edges in a backbone like this is guaranteed to form a tree. This is simply because if you have a cycle in your subgraph then removing any edge on that cycle doesn’t break connectivity or the fact that you can get from any vertex to any other (and trees are the maximal subgraphs without cycles). As such, these “backbones” are called spanning trees. “Span” here means that you can get from any vertex to any other vertex, and it suggests the connection to linear algebra that we’ll describe later, and it’s a simple property of a tree that there is a unique path between any two vertices in the tree.

An example of a spanning tree

When your edges $e \in E$ have nonnegative weights $w_e \in \mathbb{R}^{\geq 0}$, we can further ask to find a minimum cost spanning tree. The cost of a spanning tree $T$ is just the sum of its edges, and it’s important enough of a definition to offset.

Definition: A minimum spanning tree $T$ of a weighted graph $G$ (with weights $w_e \geq 0$ for $e \in E$) is a spanning tree which minimizes the quantity

$w(T) = \sum_{e \in T} w_e$

There are a lot of algorithms to find minimal spanning trees, but one that will lead us to matroids is Kruskal’s algorithm. It’s quite simple. We’ll maintain a forest $F$ in $G$, which is just a subgraph consisting of a bunch of trees that may or may not be connected. At the beginning $F$ is just all the vertices with no edges. And then at each step we add to $F$ the edge $e$ whose weight is smallest and also does not introduce any cycles into $F$. If the input graph $G$ is connected then this will always produce a minimal spanning tree.

Theorem: Kruskal’s algorithm produces a minimal spanning tree of a connected graph.

Proof. Call $F_t$ the forest produced at step $t$ of the algorithm. Then $F_0$ is the set of all vertices of $G$ and $F_{n-1}$ is the final forest output by Kruskal’s (as a quick exercise, prove all spanning trees on $n$ vertices have $n-1$ edges, so we will stop after $n-1$ rounds). It’s clear that $F_{n-1}$ is a tree because the algorithm guarantees no $F_i$ will have a cycle. And any tree with $n-1$ edges is necessarily a spanning tree, because if some vertex were left out then there would be $n-1$ edges on a subgraph of $n-1$ vertices, necessarily causing a cycle somewhere in that subgraph.

Now we’ll prove that $F_{n-1}$ has minimal cost. We’ll prove this in a similar manner to the general proof for matroids. Indeed, say you had a tree $T$ whose cost is strictly less than that of $F_{n-1}$ (we can also suppose that $T$ is minimal, but this is not necessary). Pick the minimal weight edge $e \in T$ that is not in $F_{n-1}$. Adding $e$ to $F_{n-1}$ introduces a unique cycle $C$ in $F_{n-1}$. This cycle has some strange properties. First, $e$ has the highest cost of any edge on $C$. For otherwise, Kruskal’s algorithm would have chosen it before the heavier weight edges. Second, there is another edge in $C$ that’s not in $T$ (because $T$ was a tree it can’t have the entire cycle). Call such an edge $e'$. Now we can remove $e'$ from $F_{n-1}$ and add $e$. This can only increase the total cost of $F_{n-1}$, but this transformation produces a tree with one more edge in common with $T$ than before. This contradicts that $T$ had strictly lower weight than $F_{n-1}$, because repeating the process we described would eventually transform $F_{n-1}$ into $T$ exactly, while only increasing the total cost.

$\square$

Just to recap, we defined sets of edges to be “good” if they did not contain a cycle, and a spanning tree is a maximal set of edges with this property. In this scenario, the greedy algorithm performed optimally at finding a spanning tree with minimal total cost.

## Columns of Matrices

Now let’s consider a different kind of problem. Say I give you a matrix like this one:

$\displaystyle A = \begin{pmatrix} 2 & 0 & 1 & -1 & 0 \\ 0 & -4 & 0 & 1 & 0 \\ 0 & 0 & 1 & 0 & 7 \end{pmatrix}$

In the standard interpretation of linear algebra, this matrix represents a linear function $f$ from one vector space $V$ to another $W$, with the basis $(v_1, \dots, v_5)$ of $V$ being represented by columns and the basis $(w_1, w_2, w_3)$ of $W$ being represented by the rows. Column $j$ tells you how to write $f(v_j)$ as a linear combination of the $w_i$, and in so doing uniquely defines $f$.

Now one thing we want to calculate is the rank of this matrix. That is, what is the dimension of the image of $V$ under $f$? By linear algebraic arguments we know that this is equivalent to asking “how many linearly independent columns of $A$ can we find”? An interesting consequence is that if you have two sets of columns that are both linearly independent and maximally so (adding any other column to either set would necessarily introduce a dependence in that set), then these two sets have the same size. This is part of why the rank of a matrix is well-defined.

If we were to give the columns of $A$ costs, then we could ask about finding the minimal-cost maximally-independent column set. It sounds like a mouthful, but it’s exactly the same idea as with spanning trees: we want a set of vectors that spans the whole column space of $A$, but contains no “cycles” (linearly dependent combinations), and we want the cheapest such set.

So we have two kinds of “independence systems” that seem to be related. One interesting question we can ask is whether these kinds of independence systems are “the same” in a reasonable way. Hardcore readers of this blog may see the connection quite quickly. For any graph $G = (V,E)$, there is a natural linear map from $E$ to $V$, so that a linear dependence among the columns (edges) corresponds to a cycle in $G$. This map is called the incidence matrix by combinatorialists and the first boundary map by topologists.

The map is easy to construct: for each edge $e = (v_i,v_j)$ you add a column with a 1 in the $j$-th row and a $-1$ in the $i$-th row. Then taking a sum of edges gives you zero if and only if the edges form a cycle. So we can think of a set of edges as “independent” if they don’t contain a cycle. It’s a little bit less general than independence over $\mathbb{R}$, but you can make it exactly the same kind of independence if you change your field from real numbers to $\mathbb{Z}/2\mathbb{Z}$. We won’t do this because it will detract from our end goal (to analyze greedy algorithms in realistic settings), but for further reading this survey of Oxley assumes that perspective.

So with the recognition of how similar these notions of independence are, we are ready to define matroids.

## The Matroid

So far we’ve seen two kinds of independence: “sets of edges with no cycles” (also called forests) and “sets of linearly independent vectors.” Both of these share two trivial properties: there are always nonempty independent sets, and every subset of an independent set is independent. We will call any family of subsets with this property an independence system.

Definition: Let $X$ be a finite set. An independence system over $X$ is a family $\mathscr{I}$ of subsets of $X$ with the following two properties.

1. $\mathscr{I}$ is nonempty.
2. If $I \in \mathscr{I}$, then so is every subset of $I$.

This is too general to characterize greedy algorithms, so we need one more property shared by our examples. There are a few things we do, but here’s one nice property that turns out to be enough.

Definition: A matroid $M = (X, \mathscr{I})$ is a set $X$ and an independence system $\mathscr{I}$ over $X$ with the following property:

If $A, B$ are in $\mathscr{I}$ with $|A| = |B| + 1$, then there is an element $x \in A \setminus B$ such that $B \cup \{ a \} \in \mathscr{I}$.

In other words, this property says if I have an independent set that is not maximally independent, I can grow the set by adding some suitably-chosen element from a larger independent set. We’ll call this the extension property. For a warmup exercise, let’s prove that the extension property is equivalent to the following (assuming the other properties of a matroid):

For every subset $Y \subset X$, all maximal independent sets contained in $Y$ have equal size.

Proof. For one direction, if you have two maximal sets $A, B \subset Y \subset X$ that are not the same size (say $A$ is bigger), then you can take any subset of $A$ whose size is exactly $|B| + 1$, and use the extension property to make $B$ larger, a contradiction. For the other direction, say that I know all maximal independent sets of any $Y \subset X$ have the same size, and you give me $A, B \subset X$. I need to find an $a \in A \setminus B$ that I can add to $B$ and keep it independent. What I do is take the subset $Y = A \cup B$. Now the sizes of $A, B$ don’t change, but $B$ can’t be maximal inside $Y$ because it’s smaller than $A$ ($A$ might not be maximal either, but it’s still independent). And the only way to extend $B$ is by adding something from $A$, as desired.

$\square$

So we can use the extension property and the cardinality property interchangeably when talking about matroids. Continuing to connect matroid language to linear algebra and graph theory, the maximal independent sets of a matroid are called bases, the size of any basis is the rank of the matroid, and the minimal dependent sets are called circuits. In fact, you can characterize matroids in terms of the properties of their circuits, which are dual to the properties of bases (and hence all independent sets) in a very concrete sense.

But while you could spend all day characterizing the many kinds of matroids and comatroids out there, we are still faced with the task of seeing how the greedy algorithm performs on a matroid. That is, suppose that your matroid $M = (X, \mathscr{I})$ has a nonnegative real number $w(x)$ associated with each $x \in X$. And suppose we had a black-box function to determine if a given set $S \subset X$ is independent. Then the greedy algorithm maintains a set $B$, and at every step adds a minimum weight element that maintains the independence of $B$. If we measure the cost of a subset by the sum of the weights of its elements, then the question is whether the greedy algorithm finds a minimum weight basis of the matroid.

The answer is even better than yes. In fact, the answer is that the greedy algorithm performs perfectly if and only if the problem is a matroid! More rigorously,

Theorem: Suppose that $M = (X, \mathscr{I})$ is an independence system, and that we have a black-box algorithm to determine whether a given set is independent. Define the greedy algorithm to iteratively adds the cheapest element of $X$ that maintains independence. Then the greedy algorithm produces a maximally independent set $S$ of minimal cost for every nonnegative cost function on $X$, if and only if $M$ is a matroid.

It’s clear that the algorithm will produce a set that is maximally independent. The only question is whether what it produces has minimum weight among all maximally independent sets. We’ll break the theorem into the two directions of the “if and only if”:

Part 1: If $M$ is a matroid, then greedy works perfectly no matter the cost function.
Part 2: If greedy works perfectly for every cost function, then $M$ is a matroid.

Proof of Part 1.

Call the cost function $w : X \to \mathbb{R}^{\geq 0}$, and suppose that the greedy algorithm picks elements $B = \{ x_1, x_2, \dots, x_r \}$ (in that order). It’s easy to see that $w(x_1) \leq w(x_2) \leq \dots \leq w(x_r)$. Now if you give me any list of $r$ independent elements $y_1, y_2, \dots, y_r \in X$ that has $w(y_1) \leq \dots \leq w(y_r)$, I claim that $w(x_i) \leq w(y_i)$ for all $i$. This proves what we want, because if there were a basis of size $r$ with smaller weight, sorting its elements by weight would give a list contradicting this claim.

To prove the claim, suppose to the contrary that it were false, and for some $k$ we have $w(x_k) > w(y_k)$. Moreover, pick the smallest $k$ for which this is true. Note $k > 1$, and so we can look at the special sets $S = \{ x_1, \dots, x_{k-1} \}$ and $T = \{ y_1, \dots, y_k \}$. Now $|T| = |S|+1$, so by the matroid property there is some $j$ between $1$ and $r$ so that $S \cup \{ y_j \}$ is an independent set (and $y_j$ is not in $S$). But then $w(y_j) \leq w(y_k) < w(x_k)$, and so the greedy algorithm would have picked $y_j$ before it picks $x_k$ (and the strict inequality means they’re different elements). This contradicts how the greedy algorithm runs, and hence proves the claim.

Proof of Part 2.

We’ll prove this contrapositively as follows. Suppose we have our independence system and it doesn’t satisfy the last matroid condition. Then we’ll construct a special weight function that causes the greedy algorithm to fail. So let $A,B$ be independent sets with $|A| = |B| + 1$, but for every $a \in A \setminus B$ adding $a$ to $B$ never gives you an independent set.

Now what we’ll do is define our weight function so that the greedy algorithm picks the elements we want in the order we want (roughly). In particular, we’ll assign all elements of $A \cap B$ a tiny weight we’ll call $w_1$. For elements of $B - A$ we’ll use $w_2$, and for $A - B$ we’ll use $w_3$, with $w_4$ for everything else. In a more compact notation:

We need two things for this weight function to screw up the greedy algorithm. The first is that $w_1 < w_2 < w_3 < w_4$, so that greedy picks the elements in the order we want. Note that this means it’ll first pick all of $A \cap B$, and then all of $B - A$, and by assumption it won’t be able to pick anything from $A - B$, but since $B$ is assumed to be non-maximal, we have to pick at least one element from $X - (A \cup B)$ and pay $w_4$ for it.

So the second thing we want is that the cost of doing greedy is worse than picking any maximally independent set that contains $A$ (and we know that there has to be some maximal independent set containing $A$). In other words, if we call $m$ the size of a maximally independent set, we want

$\displaystyle |A \cap B| w_1 + |B-A|w_2 + (m - |B|)w_4 > |A \cap B|w_1 + |A-B|w_3 + (m-|A|)w_4$

This can be rearranged (using the fact that $|A| = |B|+1$) to

$\displaystyle w_4 > |A-B|w_3 - |B-A|w_2$

The point here is that the greedy picks too many elements of weight $w_4$, since if we were to start by taking all of $A$ (instead of all of $B$), then we could get by with one fewer. That might not be optimal, but it’s better than greedy and that’s enough for the proof.

So we just need to make $w_4$ large enough to make this inequality hold, while still maintaining $w_2 < w_3$. There are probably many ways to do this, and here’s one. Pick some $0 < \varepsilon < 1$, and set

It’s trivial that $w_1 < w_2$ and $w_3 < w_4$. For the rest we need some observations. First, the fact that $|A-B| = |B-A| + 1$ implies that $w_2 < w_3$. Second, both $|A-B|$ and $|B-A|$ are nonempty, since otherwise the second property of independence systems would contradict our assumption that augmenting $B$ with elements of $A$ breaks independence. Using this, we can divide by these quantities to get

$\displaystyle w_4 = 2 > 1 = \frac{|A-B|(1 + \varepsilon)}{|A-B|} - \frac{|B-A|\varepsilon}{|B-A|}$

This proves the claim and finishes the proof.

$\square$

As a side note, we proved everything here with respect to minimizing the sum of the weights, but one can prove an identical theorem for maximization. The only part that’s really different is picking the clever weight function in part 2. In fact, you can convert between the two by defining a new weight function that subtracts the old weights from some fixed number $N$ that is larger than any of the original weights. So these two problems really are the same thing.

This is pretty amazing! So if you can prove your problem is a matroid then you have an awesome algorithm automatically. And if you run the greedy algorithm for fun and it seems like it works all the time, then that may be hinting that your problem is a matroid. This is one of the best situations one could possibly hope for.

But as usual, there are a few caveats to consider. They are both related to efficiency. The first is the black box algorithm for determining if a set is independent. In a problem like minimum spanning tree or finding independent columns of a matrix, there are polynomial time algorithms for determining independence. These two can both be done, for example, with Gaussian elimination. But there’s nothing to stop our favorite matroid from requiring an exponential amount of time to check if a set is independent. This makes greedy all but useless, since we need to check for independence many times in every round.

Another, perhaps subtler, issue is that the size of the ground set $X$ might be exponentially larger than the rank of the matroid. In other words, at every step our greedy algorithm needs to find a new element to add to the set it’s building up. But there could be such a huge ocean of candidates, all but a few of which break independence. In practice an algorithm might be working with $X$ implicitly, so we could still hope to solve the problem if we had enough knowledge to speed up the search for a new element.

There are still other concerns. For example, a naive approach to implementing greedy takes quadratic time, since you may have to look through every element of $X$ to find the minimum-cost guy to add. What if you just have to have faster runtime than $O(n^2)$? You can still be interested in finding more efficient algorithms that still perform perfectly, and to the best of my knowledge there’s nothing that says that greedy is the only exact algorithm for your favorite matroid. And then there are models where you don’t have direct/random access to the input, and lots of other ways that you can improve on greedy. But those stories are for another time.

Until then!

# Methods of Proof — Induction

In this final post on the basic four methods of proof (but perhaps not our last post on proof methods), we consider the proof by induction.

## Proving Statements About All Natural Numbers

Induction comes in many flavors, but the goal never changes. We use induction when we want to prove something is true about all natural numbers. These statements will look something like this:

For all natural numbers n, $1 + 2 + \dots + n = n(n+1)/2$.

Of course, there are many ways to prove this fact, but induction relies on one key idea: if we know the statement is true for some specific number $n$, then that gives us information about whether the statement is true for $n+1$. If this isn’t true about the problem, then proof by induction is hopeless.

Let’s see how we can apply it to the italicized statement above (though we haven’t yet said what induction is, this example will pave the way for a formal description of the technique). The first thing we notice is that indeed, if we know something about the first $n$ numbers, then we can just add $n+1$ to it to get the sum of the first $n+1$ numbers. That is,

$\displaystyle 1 + \dots + n + (n+1) = (1 + \dots + n) + (n+1)$

Reiterating our key point, this is how we know induction is a valid strategy: the statement written for a fixed $n$ translates naturally into the statement about $n+1$. Now suppose we know the theorem is true for $n$. Then we can rewrite the above sum as follows:

$\displaystyle 1 + \dots + n + (n+1) = \frac{n(n+1)}{2} + (n+1)$

With some algebra, we can write the left-hand side as a single fraction:

$\displaystyle 1 + \dots + (n+1) = \frac{n(n+1) + 2(n+1)}{2}$

and factoring the numerator gives

$\displaystyle 1 + \dots + (n+1) = \frac{(n+1)(n+2)}{2}$

Indeed, this is precisely what we’re looking for! It’s what happens when you replace $n$ by $n+1$ in the original statement of the problem.

At this point we’re very close to being finished. We proved that if the statement is true for $n$, then it’s true for $n+1$. And by the same reasoning, it will be true for $n+2, n+3,$ and all numbers after $n$. But this raises the obvious question: what’s the smallest number that it’s true for?

For this problem, it’s easy to see the answer is $n=1$. A mathematician would say: the statement is trivially true for $n=1$ (here trivial means there is no thinking required to show it: you just plug in $n=1$ and verify). And so by our reasoning, the statement is true for $n=2, n=3,$ and so on forever. This is the spirit of mathematical induction.

## Formal Nonsense

Now that we’ve got a taste of how to use induction in practice, let’s formally write down the rules for induction. Let’s have a statement which depends on a number $n$, and call it $p(n)$. This is written as a function because it actually is one (naively). It’s a function from the set of natural numbers to the set of all mathematical statements. In our example above, $p(n)$ was the statement that the equality $1 + \dots + n = n(n+1)/2$ holds.

We can plug in various numbers into this function, such as $p(1)$ being the statement “$1 = 1(1+1)/2$ holds,” or $p(n+1)$ being “$1 + \dots + (n+1) = (n+1)(n+1+1)/2$ holds.”

The basic recipe for induction is then very simple. Say you want to prove that $p(n)$ is true for all $n \geq 1$. First you prove that $p(1)$ is true (this is called the base case), and then you prove the implication $p(n) \to p(n+1)$ for an arbitrary $n$. Each of these pieces can be proved with any method one wishes (direct, contrapositive, contradiction, etc.). Once they are proven, we get that $p(n)$ is true for all $n$ automatically.

Indeed, we can even prove it. A rigorous proof requires a bit of extra knowledge, but we can easily understand the argument: it’s just a proof by contradiction. Here’s a quick sketch. Let $X$ be the set of all natural numbers $n$ for which $p(n)$ is false. Suppose to the contrary that $X$ is not empty. Every nonempty set of natural numbers has a smallest element, so let’s call $m$ the smallest number for which $p(m)$ is false. Now $m-1 < m$, so $p(m-1)$ must be true. But we proved that whenever $p(n)$ is true then so is $p(n+1)$, so $p(m-1 + 1) = p(m)$ is true, a contradiction.

Besides practicing proof by induction, that’s all there is to it. One more caveat is that the base case can be some number other than 1. For instance, it is true that $n! > 2^n$, but only for $n \geq 4$. In this case, we prove $p(4)$ is true, and $p(n) \to p(n+1)$ with the extra assumption that $n \geq 4$. The same inductive result applies.

Here are some exercises the reader can practice with, and afterward we will explore some variants of induction.

1. Prove that $n! > 2^n$ for all $n \geq 4$.
2. Prove that for all $n \geq 1$ the following equality holds: $1/(1 \cdot 2) + 1/(2 \cdot 3) + \dots + 1/(n \cdot (n+1)) = n/(n+1)$.
3. Prove that for every natural number $n$, a set of $n$ elements has $2^n$ subsets (including the empty subset).

This last exercise gives a hint that induction can prove more than arithmetic formulas. Indeed, if we have any way to associate a mathematical object with a number, we can potentially use induction to prove things about those objects. Unfortunately, we don’t have any mathematical objects to work with (except for sets and functions), and so we will stick primarily to proving facts about numbers.

One interesting observation about proof by induction is very relevant to programmers: it’s just recursion. That is, if we want to prove a statement $p(n)$, it suffices to prove it for $p(n-1)$ and do some “extra computation” to arrive at the statement for $p(n)$. And of course, we want to make sure the recursion terminates, so we add in the known result for $p(1)$.

## Variations on Induction, and Connections to Dynamic Programming

The first variation of induction is simultaneous induction on multiple quantities. That is, we can formulate a statement $p(n,m)$ which depends on two natural numbers independently of one another. The base case is a bit trickier, but paralleling the above remark about recursion, double-induction follows the same pattern as a two-dimensional dynamic programming algorithm. The base cases would consist of all $p(1,m)$ and all $p(n,1)$, and the inductive step to get $p(n,m)$ requires $p(n-1,m)$ and $p(n,m-1)$ (and potentially $p(n-1, m-1)$ or others; it depends on the problem).

Unfortunately, natural instances where double induction is useful (or anywhere close to necessary) are rare. Here is an example of a (tricky) but elementary example. Call

$\displaystyle C(m,n) = \frac{(2m)!(2n)!}{m!n!(m+n)!}$,

where the exclamation point denotes the factorial function. We will outline a proof that $C(m,n)$ is always an integer for all $m, n \geq 0$. If we look at the base cases, $C(0,n), C(m,0)$ (recalling that 0! = 1), we get $(2n!)/(n! n!)$, and this happens to be in the form of a binomial coefficient (here, the number of ways to choose $n!$ objects from a collection of $(2n)!$ objects), and binomial coefficients are known to be integers. Now the inductive step relies on the fact that $C(m,n-1)$ and $C(m+1, n-1)$ are both integers. If this is true then

$\displaystyle C(m,n) = 4C(m,n-1) - C(m+1, n-1)$,

which is obviously again an integer.

If we write these values in a table, we can see how the induction progresses in a “dynamic programming” fashion:

In order to fill the values in the next $n$ column (prove the statement for those values of $n$), we need to fill the entire $n-1$ column (for indeed, we rely on the inductive hypothesis for both the $m+1$ and $m$ row). But since our base case was the entire $n=0$ column, we can fill the entire table. In fact, we have just described a dynamic programming algorithm for computing the value of $C(m,n)$ in $mn$ steps. The correctness of the algorithm is indeed an inductive proof of the theorem.

Perhaps uninterestingly, this is asymptotically slower than the naive algorithm of computing $C(m,n)$ directly by computing $(2n)!, (2m)!, (n+m)!$ directly; this would take a linear number of steps in $n$, assuming $n > m$. In passing, this author wonders if, when the numbers are really large, the lack of division and multiplication in the dynamic program (multiplying by 4 using bit shifting instead) would overtake the naive algorithm. But $C(m,n)$ is certainly not interesting enough in its own right for anyone to care :)

At this point, we have noticed that we sometimes use induction and assume that many smaller instances of the statement are true. Indeed, why not inductively assume that the statement holds for all smaller $n$. This would certainly give the prover more tools to work with. Indeed, this technique is sometimes called strong induction, in the sense that we assume a stronger inductive hypothesis when we’re trying to prove $p(n+1)$. It may not be entirely obvious (especially to one well versed in the minutiae of formal logic) that this is actually equivalent to normal induction, but it is. In fact, the concept of “strong” induction is entirely pedagogical in nature. Most working mathematicians will not mention the difference in their proofs.

The last variant we’ll mention about induction is that of transfinite induction. The concept is that if you have any set $X$ which is well-ordered (essentially this means: allowing one to prove induction applies as we did earlier in the post), then we can perform induction its elements. In this way, we can “parameterize” statements by elements of an arbitrary well-ordered set, so that instead of $p(n)$ being a function from natural numbers to mathematical statements, it’s a function from $X$ to mathematical statements. One somewhat common example of when $X$ is something besides natural numbers is when we use the so-called cardinal numbers. We have already seen two distinct infinite cardinal numbers in this series: the cardinality of the integers and the cardinality of the real numbers (indeed, “cardinal number” just means a number which is the cardinality of a set). It turns out that there are many more kinds of cardinal numbers, and you can do induction on them, but this rarely shows up outside of mathematical logic.

And of course, we should close this post on an example of when induction goes wrong. For this we point the reader to our proof gallery, and the false proof that all horses are the same color. It’s quite an amusing joke, and hopefully it will stimulate the reader’s mind to recognize the pitfalls that can occur in a proof by induction.

So those are the basic four proof techniques! Fortunately for the reader, pretty much all proofs presented on this blog follow one of these four techniques. I imagine many of my readers skip over the proofs entirely (if only I could put proofs in animated gifs, with claims illustrated by grumpy cats!). So hopefully, if you have been intimidated or confused by the structure of the proofs on this blog, this will aid you in your future mathematical endeavors.  Butchering an old phrase for the sake of a bad pun, the eating of the pudding is in the proof. :)

Until next time!

# Methods of Proof — Contradiction

In this post we’ll expand our toolbox of proof techniques by adding the proof by contradiction. We’ll also expand on our knowledge of functions on sets, and tackle our first nontrivial theorem: that there is more than one kind of infinity.

## Impossibility and an Example Proof by Contradiction

Many of the most impressive results in all of mathematics are proofs of impossibility. We see these in lots of different fields. In number theory, plenty of numbers cannot be expressed as fractions. In geometry, certain geometric constructions are impossible with a straight-edge and compass. In computing theory, certain programs cannot be written. And in logic even certain mathematical statements can’t be proven or disproven.

In some sense proofs of impossibility are hardest proofs, because it’s unclear to the layman how anyone could prove it’s not possible to do something. Perhaps this is part of human nature, that nothing is too impossible to escape the realm of possibility. But perhaps it’s more surprising that the main line of attack to prove something is impossible is to assume it’s possible, and see what follows as a result. This is precisely the method of proof by contradiction:

Assume the claim you want to prove is false, and deduce that something obviously impossible must happen.

There is a simple and very elegant example that I use to explain this concept to high school students in my guest lectures.

Image you’re at a party of a hundred people, and any pair of people are either mutual friends or not mutual friends. Being a mathematical drama queen, you decide to count how many friends each person has at the party. You notice that there are two people with the same number of friends, and you wonder if it will always be the case for any party. And so the problem is born: prove or disprove that at every party of $n$ people, there exist two people with the same number of friends at the party.

If we believe this is true, and we can’t seem to find a direct proof, then we can try a proof by contradiction. That is, we assume that there are not two people with the same number of friends. Equivalently, we can assume that everybody has a distinct number of friends. Well what could the possibilities be? On one hand, if there are $n$ people at the party, then the minimum number of friends one could have is zero (if you’re quite lonely), and the maximum is $n-1$ (if you’re friends with everybody). So there are $n$ distinct numbers, and $n$ people, and everyone has to have a different number. That means someone has to have zero friends, and someone has to be friends with everybody. But this can’t possibly be true: if you’re friends with everyone (and we’re only counting mutual friendships) then nobody can be friendless. Thus, we have arrived at a contradiction, and our original assumption must have been incorrect. That is, every party has two people with the same number of friends at the party.

There are certainly other proofs of this fact (I know of a direct proof which is essentially the same proof as the one given above), and there are more mathematical ways to think about the problem. But this is a wonderful example of a proof which requires little else than the method of contradiction.

## A Reprise on Truth Tables, and More Examples

Just as with our post on contrapositive implication, we can analyze proof by contradiction from the standpoint of truth tables. Recall the truth table for an implication $p \to q$:

p  q  p->q
T  T   T
T  F   F
F  T   T
F  F   T

We notice that an implication can only be false if the hypothesis $p$ is true and the consequence $q$ is false. This is the motivation for a proof by contradiction: if we show this case can’t happen, then there’s no other option: the statement $p \to q$ must be true. In other words, if supposing “p and not q” is true implies something which we know to be false, then by the very same truth table argument it must be that either “q” is true or “p” is false. In any of these cases “p implies q” is true.

But all of this truth table juggling really takes us away from the heart of the method. Let’s do some proofs.

First, we will prove that the square root of 2 is not a rational number. That is, we are proving the statement that if $x$ is a number such that $x^2 = 2$, then it cannot be true that $x = p/q$ where $p,q$ are integers.

Suppose to the contrary (this usually marks the beginning of a proof by contradiction) that $x = p/q$ is a ratio of integers. Then we can square both sides to get $2 = x^2 = p^2 / q^2$, and rearrange to arrive at $2q^2 = p^2$. Now comes the tricky part: if a number is a divisor of $p$, then its square must divide $p^2$; this is true of all square numbers. In particular, it must be the case that the largest power of 2 dividing any square number is even (and $2^0$ counts as an even power). Now in the equation $2q^2 = p^2$ the right hand side is a square, so the largest power of two dividing it is even, and the right hand side is two times a square, so the largest power of 2 dividing it is odd (2 times an even power of 2 gives an odd power of two). But the two sides are equal! They can’t possibly have different largest powers of 2 dividing them. So we have arrived at a contradiction, and it must not be the case that $x$ is rational.

This is quite a nice result, and a true understanding of the proof comes when you see why it fails for numbers which actually do have rational square roots (for instance, try it for the square root of 9 or 36/25). But the use of the method is clear: we entered a magical fairy land where the square root of 2 is a rational number, and by using nothing but logical steps, we proved that this world is a farce. It cannot exist.

Often times the jump from “suppose to the contrary” to the contradiction requires a relatively small piece of insight, but in other times it is quite large. In our example above, the insight was related to divisors (or prime factorizations) of a number, and these are not at all as obvious to the layman as our “having no friends” contradiction earlier.

For instance, here is another version of the square root of two proof, proved by contradiction, but this time using geometry. Another example is on tiling chessboards with dominoes (though the application of the proof by contradiction in this post is more subtle; can you pick out exactly when it’s used?). Indeed, most proofs of the fundamental theorem of algebra (these are much more advanced: [1] [2] [3] [4]) follow the same basic recipe: suppose otherwise, and find a contradiction.

Instead of a obviously ridiculous statement like “1 = 0″, often times the “contradiction” at the end of a proof will contradict the original hypothesis that was assumed. This happens in a famous proof that there are infinitely many prime numbers.

Indeed, if we suppose that there are finitely many prime numbers, we can write them all down: $p_1 , \dots, p_n$. That is, we are assuming that this is a list of all prime numbers. Since the list is finite, we can multiply them all together and add 1: let $q = p_1 \dots p_n + 1$. Indeed, as the reader will prove in the exercises below, every number has a prime divisor, but it is not hard to see that no $p_i$ divides $q$. This is because no matter what some number $x$ is, no number except 1 can divide both $x$ and $x-1$ (one can prove this fact by contradiction if it is not obvious), and we already know that all the $p_i$ divide $q-1$ . So $q$ must have some prime divisor which was not in the list we started with. This contradicts that we had a complete list of primes to begin with. And so there must be infinitely many primes.

Here are some exercises to practice the proof by contradiction:

1. Prove that the base 2 logarithm of 3 is irrational.
2. More generally that $\log_a(b)$ is irrational if there is any prime $p$ dividing $a$ but not $b$, or vice versa.
3. Prove the fundamental theorem of arithmetic, that every natural number $n \geq 2$ is a product of primes (hint: inspect the smallest failing example).

## A Few More Words on Functions and Sets

Last time we defined what it means for a function $f: X \to Y$ on sets to be injective: different things in $X$ get mapped to different things in $Y$. Indeed, there is another interesting notion called surjectivity, which says that $f$ “hits” everything in $Y$ by something in $X$.

Definition: A function $f: X \to Y$ is surjective if for every element $y \in Y$ there is an $x \in X$ for which $f(x) = y$. A surjective function is called a surjection. A synonym often used in place of surjective is onto.

For finite sets, we use surjections to prove something nice about the sets it involves. If $f:X \to Y$ is a surjection, then $X$ has at least as many elements as $Y$. The reader can prove this easily by contradiction. In our previous post we proved an analogous proposition for injective functions: if $f: X \to Y$ is injective, then there are at least as many elements of $Y$ as there are of $X$. If we combine the two notions, we can see that $X$ and $Y$ have exactly the same size.

Definition: A function $f: X \to Y$ which is both injective and surjective is called a bijection. The adjectival form of bijection is bijective.

So for finite sets, if there exists a bijection $X \to Y$, then $X$ and $Y$ have the same number of elements. The converse is also true, if two finite sets have the same size one can make a bijection between them (though a strictly formal proof of this would require induction, which we haven’t covered yet). The main benefit of thinking about size this way is that it extends to infinite sets!

Definition: Two arbitrary sets $X,Y$ are said to have the same cardinality if there exists a bijection $f : X \to Y$. If there is a bijection $f: \mathbb{N} \to X$ then $X$ is said to have countably infinite cardinality, or simply countably infinite. If no such bijection exists (and $X$ is not a finite set), then $X$ is said to be uncountably infinite.

So we can say two infinite sets have the same cardinality if we can construct a bijection between them. For instance, we can prove that the even natural numbers have the same cardinality as the regular natural numbers. If $X$ is the set of even natural numbers, just construct a function $\mathbb{N} \to X$ by sending $x \mapsto 2x$. This is manifestly surjective and injective (one can prove it with whatever method one wants, but it is obviously true). A quick note on notation: when mathematicians want to define a function without giving it a name, they use the “maps to” arrow $\mapsto$. The reader can think of this as the mathematician’s version of lambda expression. So the above map would be written in python: lambda x: 2*x.

So we have proved, as curious as it sounds to say it, that there are just as many even numbers as not even numbers. Even more impressive, one can construct a bijection between the natural numbers and the rational numbers. Mathematicians denote the latter by $\mathbb{Q}$, and typically this proof is done by first finding a bijection from $\mathbb{N} \to \mathbb{Z}$ and then from $\mathbb{Z} \to \mathbb{Q}$. We are implicitly using the fact that a composition of two bijections is a bijection. The diligent reader has already proved this for injections, so if one can also prove it for surjections, by definition it will be satisfied for bijections.

## Diagonalization, and a Non-Trivial Theorem

We now turn to the last proof of this post, and our first non-trivial theorem: that there is no bijection between the set of real numbers and the set of natural numbers. Before we start, we should mention that calling this theorem ‘non-trivial’ might sound insulting to the non-mathematician; the reader has been diligently working to follow the proofs in these posts and completing exercises, and they probably all feel non-trivial. In fact, mathematicians don’t use trivial with the intent to insult (most of the time) or to say something is easy or not worth doing. Instead, ‘trivial’ is used to say that a result follows naturally, that it comes from nothing but applying the definitions and using the basic methods of proof. Of course, since we’re learning the basic methods of proof nothing can really be trivial, but if we say a theorem is non-trivial that means the opposite: there is some genuinely inspired idea that sits at the focal point of the proof, more than just direct or indirect inference. Even more, a proof is called “highly non-trivial” if there are multiple novel ideas or a menagerie of complicated details to keep track of.

In any case, we have to first say what the real numbers are. Instead we won’t actually work with the entire set of real numbers, but with a “small” subset: the real numbers between zero and one. We will also view these numbers in a particular representation that should be familiar to the practicing programmer.

Definition: The set of $[0,1]$ is the set of all infinite sequences of zeroes and ones, interpreted as the set of all binary decimal expansions of numbers between zero and one.

If we want to be rigorous, we can define an infinite sequence to either be an infinite tuple (falling back on our definition of a tuple as a set), or we can define it to be a function $f : \mathbb{N} \to \left \{ 0, 1 \right \}$. Taking the latter view, we add one additional piece of notation:

Definition: An infinite binary sequence $(b_i) = (b_1, b_2, \dots)$ is a function $b : \mathbb{N} \to \left \{ 0, 1 \right \}$ where we denote by $b_i$ the value $b(i)$.

So now we can state the theorem.

Theorem: The set $[0,1]$ of infinite binary sequences is uncountably infinite. That is, there is no bijection $\mathbb{N} \to [0,1]$.

The proof, as we said, is non-trivial, but it starts off in a familiar way: we assume there is such a bijection. Suppose to the contrary that $f : \mathbb{N} \to [0,1]$ is a bijection. Then we can list the values of $f$ in a table. Since we want to use $b_i$ for all of the values of $f$, we will call

$\displaystyle f(n) = (b_{n,i}) = b_{n,1}, b_{n,2}, \dots$

This gives us the following infinite table:

$\displaystyle \begin{matrix} f(1) &=& b_{1,1}, & b_{1,2}, & \dots \\ f(2) &=& b_{2,1}, & b_{2,2}, & \dots \\ f(3) &=& b_{3,1}, & b_{3,2}, & \dots \\ \vdots & & \vdots & & \end{matrix}$

Now here is the tricky part. We are going to define a new binary sequence which we can guarantee does not show up in this list. This will be our contradiction, because we assumed at first that this list consisted of all of the binary sequences.

The construction itself is not so hard. Start by taking $c_i = b_{i,i}$ for all $i$. That is, we are using all of the diagonal elements of the table above. Now take each $c_i$ and replace it with its opposite (i.e., flip each bit in the sequence, or equivalently apply $b \mapsto 1-b$ to each entry). The important fact about this new sequence is it differs from every entry in this table. By the way we constructed it, no matter which $lateex n$ one chooses, this number differs from the table entry $f(n)$ at digit $n$ (and perhaps elsewhere). Because of this, it can’t occur as an entry in the table. So we just proved our function $f$ isn’t surjective, contradicting our original hypothesis, and proving the theorem.

The discovery of this fact was an important step forward in the history of mathematics. The particular technique though, using the diagonal entries of the table and changing each one, comes with a name of its own: the diagonalization argument. It’s quite a bit more specialized of a proof technique than, say, the contrapositive implication, but it shows up in quite a range of mathematical literature (for instance, diagonalization is by far the most common way to prove that the Halting problem is undecidable). It is worth noting diagonalization was not the first known way to prove this theorem, just the cleanest.

The fact itself has interesting implications that lends itself nicely to confusing normal people. For instance, it implies not only that there is more than one kind of infinity, but that there are an infinity of infinities. Barring a full discussion of how far down the set-theory rabbit hole one can go, we look forward to next time, when we meet the final of the four basic methods of proof: proof by induction.

Until then!

# Methods of Proof — Contrapositive

In this post we’ll cover the second of the “basic four” methods of proof: the contrapositive implication. We will build off our material from last time and start by defining functions on sets.

## Functions as Sets

So far we have become comfortable with the definition of a set, but the most common way to use sets is to construct functions between them. As programmers we readily understand the nature of a function, but how can we define one mathematically? It turns out we can do it in terms of sets, but let us recall the desired properties of a function:

• Every input must have an output.
• Every input can only correspond to one output (the functions must be deterministic).

One might try at first to define a function in terms of subsets of size two. That is, if $A, B$ are sets then a function $f: A \to B$ would be completely specified by

$\displaystyle \left \{ \left \{ x, y \right \} : x \in A, y \in B \right \}$

where to enforce those two bullets, we must impose the condition that every $x \in A$ occurs in one and only one of those subsets. Notationally, we would say that $y = f(x)$ means $\left \{ x, y \right \}$ is a member of the function. Unfortunately, this definition fails miserably when $A = B$, because we have no way to distinguish the input from the output.

To compensate for this, we introduce a new type of object called a tuple. A tuple is just an ordered list of elements, which we write using round brackets, e.g. $(a,b,c,d,e)$.

As a quick aside, one can define ordered tuples in terms of sets. We will leave the reader to puzzle why this works, and generalize the example provided:

$\displaystyle (a,b) = \left \{ a, \left \{ a, b \right \} \right \}$

And so a function $f: A \to B$ is defined to be a list of ordered pairs where the first thing in the pair is an input and the second is an output:

$\displaystyle f = \left \{ (x, y) : x \in A, y \in B \right \}$

Subject to the same conditions, that each $x$ value from $A$ must occur in one and only one pair. And again by way of notation we say $y = f(x)$ if the pair $(x,y)$ is a member of $f$ as a set. Note that the concept of a function having “input and output” is just an interpretation. A function can be viewed independent of any computational ideas as just a set of pairs. Often enough we might not even know how to compute a function (or it might be provably uncomputable!), but we can still work with it abstractly.

It is also common to call functions “maps,” and to define “map” to mean a special kind of function (that is, with extra conditions) depending on the mathematical field one is working in. Even in other places on this blog, “map” might stand for a continuous function, or a homomorphism. Don’t worry if you don’t know these terms off hand; they are just special cases of functions as we’ve defined them here. For the purposes of this series on methods of proof, “function” and “map” and “mapping” mean the same thing: regular old functions on sets.

## Injections

One of the most important and natural properties of a function is that of injectivity.

Definition: A function $f: A \to B$ is an injection if whenever $a \neq a'$ are distinct members of $A$, then $f(a) \neq f(a')$. The adjectival version of the word injection is injective.

As a quick side note, it is often the convention for mathematicians to use a capital letter to denote a set, and a lower-case letter to denote a generic element of that set. Moreover, the apostrophe on the $a'$ is called a prime (so $a'$ is spoken, “a prime”), and it’s meant to denote a variation on the non-prime’d variable $a$ in some way. In this case, the variation is that $a' \neq a$.

So even if we had not explicitly mentioned where the $a, a'$ objects came from, the knowledgeable mathematician (which the reader is obviously becoming) would be reasonably certain that they come from $A$. Similarly, if I were to lackadaisically present $b$ out of nowhere, the reader would infer it must come from $B$.

One simple and commonly used example of an injection is the so-called inclusion function. If $A \subset B$ are sets, then there is a canonical function representing this subset relationship, namely the function $i: A \to B$ defined by $i(a) = a$. It should be clear that non-equal things get mapped to non-equal things, because the function doesn’t actually do anything except change perspective on where the elements are sitting: two nonequal things sitting in $A$ are still nonequal in $B$.

Another example is that of multiplication by two as a map on natural numbers. More rigorously, define $f: \mathbb{N} \to \mathbb{N}$ by $f(x) = 2x$. It is clear that whenever $x \neq y$ as natural numbers then $2x \neq 2y$. For one, $x, y$ must have differing prime factorizations, and so must $2x, 2y$ because we added the same prime factor of 2 to both numbers. Did you catch the quick proof by direct implication there? It was sneaky, but present.

Now the property of being an injection can be summed up by a very nice picture:

A picture example of an injective function.

The arrows above represent the pairs $(x,f(x))$, and the fact that no two arrows end in the same place makes this function an injection. Indeed, drawing pictures like this can give us clues about the true nature of a proposed fact. If the fact is false, it’s usually easy to draw a picture like this showing so. If it’s true, then the pictures will support it and hopefully make the proof obvious. We will see this in action in a bit (and perhaps we should expand upon it later with a post titled, “Methods of Proof — Proof by Picture”).

There is another, more subtle concept associated with injectivity, and this is where its name comes from. The word “inject” gives one the mental picture that we’re literally placing one set $A$ inside another set $B$ without changing the nature of $A$. We are simply realizing it as being inside of $B$, perhaps with different names for its elements. This interpretation becomes much clearer when one investigates sets with additional structure, such as groups, rings, or topological spaces. Here the word “injective mapping” much more literally means placing one thing inside another without changing the former’s structure in any way except for relabeling.

In any case, mathematicians have the bad (but time-saving) habit of implicitly identifying a set with its image under an injective mapping. That is, if $f :A \to B$ is an injective function, then one can view $A$ as the same thing as $f(A) \subset B$. That is, they have the same elements except that $f$ renames the elements of $A$ as elements of $B$. The abuse comes in when they start saying $A \subset B$ even when this is not strictly the case.

Here is an example of this abuse that many programmers commit without perhaps noticing it. Suppose $X$ is the set of all colors that can be displayed on a computer (as an abstract set; the elements are “this particular green,” “that particular pinkish mauve”). Now let $Y$ be the set of all finite hexadecimal numbers. Then there is an obvious injective map from $X \to Y$ sending each color to its 6-digit hex representation. The lazy mathematician would say “Well, then, we might as well say $X \subset Y$, for this is the obvious way to view $X$ as a set of hexadecimal numbers.” Of course there are other ways (try to think of one, and then try to find an infinite family of them!), but the point is that this is the only way that anyone really uses, and that the other ways are all just “natural relabelings” of this way.

The precise way to formulate this claim is as follows, and it holds for arbitrary sets and arbitrary injective functions. If $g, g': X \to Y$ are two such ways to inject $X$ inside of $Y$, then there is a function $h: Y \to Y$ such that the composition $hg$ is precisely the map $g'$. If this is mysterious, we have some methods the reader can use to understand it more fully: give examples for simplified versions (what if there were only three colors?), draw pictures of “generic looking” set maps, and attempt a proof by direct implication.

## Proof by Contrapositive

Often times in mathematics we will come across a statement we want to prove that looks like this:

If X does not have property A, then Y does not have property B.

Indeed, we already have: to prove a function $f: X \to Y$ is injective we must prove:

If x is not equal to y, then f(x) is not equal to f(y).

A proof by direct implication can be quite difficult because the statement gives us very little to work with. If we assume that $X$ does not have property $A$, then we have nothing to grasp and jump-start our proof. The main (and in this author’s opinion, the only) benefit of a proof by contrapositive is that one can turn such a statement into a constructive one. That is, we can write “p implies q” as “not q implies not p” to get the equivalent claim:

If Y has property B then X has property A.

This rewriting is called the “contrapositive form” of the original statement. It’s not only easier to parse, but also probably easier to prove because we have something to grasp at from the beginning.

To the beginning mathematician, it may not be obvious that “if p then q” is equivalent to “if not q then not p” as logical statements. To show that they are requires a small detour into the idea of a “truth table.”

In particular, we have to specify what it means for “if p then q” to be true or false as a whole. There are four possibilities: p can be true or false, and q can be true or false. We can write all of these possibilities in a table.

p  q
T  T
T  F
F  T
F  F

If we were to complete this table for “if p then q,” we’d have to specify exactly which of the four cases correspond to the statement being true. Of course, if the p part is true and the q part is true, then “p implies q” should also be true. We have seen this already in proof by direct implication. Next, if p is true and q is false, then it certainly cannot be the case that truth of p implies the truth of q. So this would be a false statement. Our truth table so far looks like

p  q  p->q
T  T   T
T  F   F
F  T   ?
F  F   ?

The next question is what to do if the premise p of “if p then q” is false. Should the statement as a whole be true or false? Rather then enter a belated philosophical discussion, we will zealously define an implication to be true if its hypothesis is false. This is a well-accepted idea in mathematics called vacuous truth. And although it seems to make awkward statements true (like “if 2 is odd then 1 = 0″), it is rarely a confounding issue (and more often forms the punchline of a few good math jokes). So we can complete our truth table as follows

p q  p->q
T T   T
T F   F
F T   T
F F   T

Now here’s where contraposition comes into play. If we’re interested in determining when “not q implies not p” is true, we can add these to the truth table as extra columns:

p  q  p->q  not q   not p   not q -> not p
T  T   T      F       F           T
T  F   F      T       F           F
F  T   T      F       T           T
F  F   T      T       T           T

As we can see, the two columns corresponding to “p implies q” and “not q implies not p” assume precisely the same truth values in all possible scenarios. In other words, the two statements are logically equivalent.

And so our proof technique for contrapositive becomes: rewrite the statement in its contrapositive form, and proceed to prove it by direct implication.

## Examples and Exercises

Our first example will be completely straightforward and require nothing but algebra. Let’s show that the function $f(x) = 7x - 4$ is injective. Contrapositively, we want to prove that if $f(x) = f(x')$ then $x = x'$. Assuming the hypothesis, we start by supposing $7x - 4 = 7x' - 4$. Applying algebra, we get $7x = 7x'$, and dividing by 7 shows that $x = x’$ as desired. So $f$ is injective.

This example is important because if we tried to prove it directly, we might make the mistake of assuming algebra works with $\neq$ the same way it does with equality. In fact, many of the things we take for granted about equality fail with inequality (for instance, if $a \neq b$ and $b \neq c$ it need not be the case that $a \neq c$). The contrapositive method allows us to use our algebraic skills in a straightforward way.

Next let’s prove that the composition of two injective functions is injective. That is, if $f: X \to Y$ and $g: Y \to Z$ are injective functions, then the composition $gf : X \to Z$  defined by $gf(x) = g(f(x))$ is injective.

In particular, we want to prove that if $x \neq x'$ then $g(f(x)) \neq g(f(x'))$. Contrapositively, this is the same as proving that if $g(f(x)) = g(f(x'))$ then $x=x'$. Well by the fact that $g$ is injective, we know that (again contrapositively) whenever $g(y) = g(y')$ then $y = y'$, so it must be that $f(x) = f(x')$. But by the same reasoning $f$ is injective and hence $x = x'$. This proves the statement.

This was a nice symbolic proof, but we can see the same fact in a picturesque form as well:

A composition of two injections is an injection.

If we maintain that any two arrows in the diagram can’t have the same head, then following two paths starting at different points in $X$ will never land us at the same place in $Z$. Since $f$ is injective we have to travel to different places in $Y$, and since $g$ is injective we have to travel to different places in $Z$. Unfortunately, this proof cannot replace the formal one above, but it can help us understand it from a different perspective (which can often make or break a mathematical idea).

Expanding upon this idea we give the reader a challenge: Let $A, B, C$ be finite sets of the same size. Prove or disprove that if $f: A \to B$ and $g: B \to C$ are (arbitrary) functions, and if the composition $gf$ is injective, then both of $f, g$ must be injective.

Another exercise which has a nice contrapositive proof: prove that if $A,B$ are finite sets and $f:A \to B$ is an injection, then $A$ has at most as many elements as $B$. This one is particularly susceptible to a “picture proof” like the one above. Although the formal the formal name for the fact one uses to prove this is the pigeonhole principleit’s really just a simple observation.

Aside from inventing similar exercises with numbers (e.g., if $ab$ is odd then $a$ is odd or $b$ is odd), this is all there is to the contrapositive method. It’s just a direct proof disguised behind a fact about truth tables. Of course, as is usual in more advanced mathematical literature, authors will seldom announce the use of contraposition. The reader just has to be watchful enough to notice it.

Though we haven’t talked about either the real numbers $\mathbb{R}$ nor proofs of existence or impossibility, we can still pose this interesting question: is there an injective function from $\mathbb{R} \to \mathbb{N}$? In truth there is not, but as of yet we don’t have the proof technique required to show it. This will be our next topic in the series: the proof by contradiction.

Until then!