The problem was to find the largest integer which divides every member of the fo llowing sequence:

$1^{5} - 1, 2^{5} - 2, 3^{5} - 3, \dots n^{5} - n .$

The solution depends only on a little algebra and some clear mathematical thinking.

Pierre Geoghegan, age 17, Tarbert Comprehensive, Ireland, Prateek Mehrotra, age 14, Riccarton High School, Christchurch, New Zealand and Vassil Vassilev, age 16, from Lawnswood Sixth Form, Leeds started by taking small values of $n$ , usually a good way to begin. This solution comes from Arun Iyer, age 17, S.I.A High School and Junior College, India, and they all found the answer which is 30.

Given the sequence $1^{5} - 1, 2^{5} - 2, 3^{5} - 3, \dots n^{5} - n$ we see that

$n^{5} - n = n (n^{4} - 1) = n (n - 1) (n + 1) (n^{2} + 1)$

and it is quite easy to see that $n (n - 1) (n + 1) (n^{2} + 1)$ is divisible by 2, 3 and 5 for all values of $n$ . As $n$ , $(n - 1)$ and $(n + 1)$ are three consecutive integers their product must be divisible by 2 and by 3. If none of these numbers is divisible by 5 then $n$ is either of the form $5 k + 2$ or $5 k + 3$ for some integer $k$ and in both of these cases we can check that $n^{2} + 1$ is divisible by 5. Since 2, 3 and 5 are coprime therefore $n^{5} - n$ is divisible by 2*3*5 i.e by 30.

Since the second term of the sequence is $2^{5} - 2 = 30$ therefore the divisor cannot be greater than 30. Therefore 30 is the largest number that d ivides each member of the sequence.