The basics of induction
The most obvious interesting thing about the natural numbers is that it’s natural to start listing them, one after the other: This, of course, is how counting works.
It turns out that this way of thinking about the integers gives us a very powerful tool for proving things one integer at a time: the principle of mathematical induction, usually known to mathematicians simply as induction.
Informally, I like to think of the following example:
If I can reach the bottom (rung number zero?) of a ladder,
and if I’m on any rung I can reach the next rung up,
then I can reach any rung on the ladder.
Why, for example, can I reach the fourth rung? One can imagine a detailed proof of this, as follows:
I can reach rung zero;
Because I can reach rung zero, I can reach rung one;
Because I can reach rung one, I can reach rung two;
Because I can reach rung two, I can reach rung three;
Because I can reach rung three, I can reach rung four.
The connection with counting is obvious: our proof visibly counts up to four.
Writing that out was okay, but you are probably glad I didn’t write out a proof that we could reach the hundred and seventy-eighth rung. I suppose that we could do so, writing “and so on” at some point: but that’s a little vague (what about situations where it isn’t obvious what “and so on” means)?
It’s helpful to have a way which isn’t vague.
So here’s a formal version:
Definition: [Induction] Let be a statement that depends on a natural number . Then, if
the statement is true, and
for all , if is true, then is true,
then the statement is true for all .
Here are some useful words:
We call part () the base case, and part () the induction step. These words agree quite well with our mental picture of a ladder!
When we are trying to prove the induction step we refer to as the induction hypothesis.
Example of induction
We’ll prove many things by induction in this course, but this is one:
For any natural number , we have the following formula for the sum of the first positive integers:
Let be the statement above for some particular .
So is the statement that says , and is the statement that Notice that is not a number, it’s a statement.
We will prove , which says that for all by induction.
For our base case, says that the sum of no integers at all is , which is true, as the sum of no integers is zero.
Now we will do our induction step, proving for all . Suppose is true: we need to show that is true.
The statement tells us that We need to prove , which would say that Now note that This is exactly the statement , which is what we needed for the induction step, and that completes the proof.
You may know other ways of proving that. (I can think of a few.) But I hope you’re impressed with this as a strong potential method for proving identities.
Nonexamples of induction
Let’s now try proving some completely false statements using induction. The plan is (of course) not to succeed, but to understand where we need to be careful.
Clearly this statement is complete and utter rubbish.
If you believe that induction is a reliable method of proof (and I do, and I hope you do too), then it had better be the case that we’re not using induction correctly.
Anyway, here’s an induction “proof”. Suppose that for some . We’ll prove that . But we have This completes the proof.
What’s the problem with the argument above?
There’s no base case.
If you don’t have a base case, such as , then it’s of no use to prove that for all . It’s no use to be able to climb a ladder if the bottom of the ladder is unreachable.
Here’s another, more subtle example:
Again, we find ourselves hoping very strongly that there’s a mistake in the use of induction in what follows. I’ll write it out and we can see if we can spot it.
In order to do this, we’ll let be the statement “Given any horses, all of them have the same colour”. We’ll prove for all by induction: that will give us what we want, because we can take to be the number of horses in the world.
We’ll take as the base case of the induction. This is the statement “Given any one horse, all of them have the same colour”: this is obviously true.
Now we’ll prove the induction step. We will assume that is true (“given any horses, all of them have the same colour”): our job is to prove that is true (“given any horses, all of them have the same colour”).
So suppose we have horses. Name two of them Alice and Zebedee.
Excluding Alice, there are horses, which all have the same colour, by the induction hypothesis. So all the horses except Alice have the same colour as Zebedee.
Also, excluding Zebedee, there are horses, which all have the same colour, again by the induction hypothesis. So all the horses except Zebedee have the same colour as Alice.
Hence all the horses except Alice and Zebedee have the same colour as both Alice and Zebedee, which says that all the horses have the same colour. That ends the proof.
What’s wrong with this?
The particular case doesn’t work.
I find this surprisingly subtle.
In fact, it’s a parody of a valid style of argument. If it is the case that any two things are the same, then we could prove using exactly this method that they’re all the same. In fact, this is something you already know, since “all are alike” and “no two differ” are synonymous phrases.
There are other techniques which use the same idea of induction, but not quite the same formal principle as I’ve written out above.
We can start with a base case which isn’t . For example, if
is true, and
implies for all ,
then is true for all .
Perhaps you want to think of that as saying “if have a door which leads to the fifteenth rung of a ladder, and you know how to climb ladders, then you can get to every rung above the fifteenth”.
Actually, this is really just ordinary induction in disguise.
Indeed, if we define to be , then proving for all by induction is the same as proving for all integers .
Perhaps you want to think of that as “ignoring all the bits of the ladder below the fifteenth rung, imagining the ladder starts outside your door, and starting counting rungs from there”. Or perhaps you’re bored of the ladder analogy now.
Actually, you should have been prepared for this variant: my induction proof that “all horses have the same colour” started with , not . (Okay, that proof was wrong. But there was nothing wrong with that bit of the proof: there’s nothing wrong with induction starting from . It was something else that was wrong).