How many positive integers are there for which is an integer?
Solution
The answer is 6. We will show that the desired integers are precisely the odd positive integers less than or equal to 11.
We observe that and , so we are looking for the positive integers for which the fraction is an integer.
We first observe that if is even this never happens: indeed the numerator is even, but not divisible by 4, since and both leave remainder 1 upon division by 4 (to prove this fact it suffices to observe that is a product of two even numbers, and is therefore divisible by 4; similarly for ), and so their sum leaves remainder 2 upon division by 4.
When is odd, instead, we can use the well-known factorization for a sum of two odd powers with the same exponent to obtain
The number in the second parenthesis is the algebraic sum of odd terms, and is therefore odd. The product just written is therefore divisible by if and only if is. This happens if and only if , that is, for , values which give the 6 desired solutions.