Proof by Induction

Induction Hypothesis

Proof by induction is a very useful technique for proving that a hypothesis is true for all integers starting from some small integer (generally 0 or 1). The hypothesis is called the induction hypothesis, which we will abbreviate as IH. We will say IH(0) to refer to the induction hypothesis for the integer 0, IH(1) for the integer 1, and IH(n) for the integer n.

In principle, we could prove the cases of the induction hypothesis one at a time. In other words, prove that IH(0) is true, then prove that IH(1) is true, then IH(2), etc. However, since our goal is to prove the induction hypothesis for all integers (greater than our starting point), we will never finish, since there is an infinite number of such integers!

Proof By Induction

Instead, we split up the proof into two steps.

The Basis Step. In this step we prove that the induction hypothesis is true for the first integer. For example, if we’re starting from 0, then we prove that IH(0) is true.

The Induction Step. In this step we prove that if IH(n) is true, then IH(n+1) must be true.

It should be easy to see how these two steps can be used to “cover” all integers starting from the initial integer up to any arbitrary integer n. The basis step proves that the induction hypothesis is true for the first integer. Then, by applying the induction step some finite number of times, we can eventually prove IH(n) is true for any integer n. For example, proving the basis step and the induction step allows us to show that IH(3) is true:

IH(0) is true (by the Basis Step)

IH(0) implies IH(1) (by the Induction Step)

IH(1) implies IH(2) (by the Induction Step)

IH(2) implies IH(3) (by the Induction Step)

We can construct such a proof for any n, so that means that the induction hypothesis must be true for all values of n.

Proving the Induction Step

Proving the induction step is typically the more interesting of the two steps. Here is the general approach.

(1) State IH(n) and IH(n+1). The statement of IH(n+1) is called the expectation, and is extremely important.

(2) Show why if IH(n) is true, then IH(n+1) must also be true. A very common approach is to define a recurrence which, based on the nature of the property you are trying to prove, shows how what is true for n+1 is related to what is true for n. Because the recurrence establishes a link from n to n+1, it can be used to prove the induction step.

An Example

A concrete example will show how this works. We will prove that the sum of the integers from 1 to n, which we will refer to as f(n), is (n(n+1)) / 2. So, the general form of our induction hypothesis is

f(n) = (n(n+1)) / 2

Basis Step. IH(1) is f(1) = 1. Since the series of integers from 1 to 1 is simply 1, this is true trivially.

Induction Step. First, we will state IH(n) and IH(n+1):

IH(n) is f(n) = (n(n+1)) / 2

IH(n+1) is f(n+1) = ((n+1)(n+2))/2

Our statement of IH(n +1) is the expectation. The expectation tells us what we expect to be true if the induction hypothesis is true for n+1. We obtained the expectation by simply substuting n+1 for n in the equation we are trying to prove correct.

Now the critical part: we need to show how what is true for n+1 is related to what is true for n. Because f(n) is the sum of the integers 1..n, then adding (n+1) to this sum will obviously result in the sum of the integers 1..n+1. We can state this observation as a recurrence:

f(n+1) = f(n) + (n+1)

The recurrence simply states that the sum of the integers 1..n+1 is equal to the sum of the integers 1..n + (n+1).

Now that we have established the recurrence, we can prove the induction step. In the induction step, we show that if or is true, then IH(n+1) must be true. (Note that we are not proving that either IH(n) or IH(n+1) is true outright. We are simply proving that if we assume that IH(n) true, then IH(n+1) must also be true.)

Once we have the recurrence, it is very simple to provie the induction step: we simply expand the occurrence of f(n). We are allowed to assume that IH(n) is true:

IH(n) is f(n) = (n(n+1)) / 2

So, the expanded recurrence is

f(n+1) = (n(n+1))/2 + (n+1)

Now all we need to do is rearrange the right hand side of the equation so that it matches the expectation (which is what we expect if IH(n+1) is true):

f(n+1) = (n(n+1))/2 + 2(n+1)/2      multiply (n+1) by 2/2

f(n+1) = (n^2+n)/2 + (2n+2)/2      expand terms

f(n+1) = (n^2+3n+2)/2      combine terms

f(n+1) = ((n+1)(n+2))/2      factor polynomial

The expanded recurrence now exactly matches the expectation, proving that if IH(n) is true, then IH(n+1) must also be true.

Ta-da!


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