‘Do all odd square numbers greater than 1 have 3 factors?’

I love this question. Let me explain why, and the train of thought that it can generate.

Children usually start by generating the odd square numbers, identifying that square numbers have an odd number of factors (1, the number and the square root). This helps to underline the uniqueness of square numbers. Nothing new here, really.

So children tend to identify the first two or three odd squares greater than one (9, 25 and 49) and realise that these numbers only have 3 factors. This, I tend to find, is enough evidence to convince most children that the answer to the question must be ‘yes’.

However, this is of course a false presumption, and by making it children realise a crucial mathematical principle: that finding examples to support a theory is not the same as finding a proof. There must be reasoning as well as examples to generate a proof!

So I then ask the children to consider 81. And they soon realise that it is also divisible by 3. What’s happened to the pattern? And why?

Children then investigate further examples, noting that 5 of the first 6 odd square numbers greater than 1 (9, 25, 49, 121, 169) have only 3 factors. But then 225, the square of 15, has 9 factors! There must be some logic here, and of course there is. I ask the children to consider the square roots of each number, as underlined below:

After much discussion and deliberation, and a healthy dollop of struggle, someone makes the breakthrough: the numbers with prime square roots have 3 factors; other numbers can be further divided by the factors of the square.

But does this pattern continue for all odd squares? And why was 1 excluded from the list?

It’s an amazing question for exploring the very nature of the properties of number, and particularly primes and squares. And, in my opinion, for experiencing the joy and beauty of mathematics!