Which number does not belong in the following series 15 2 8 13 16?

15

* * * * *

Utter nonsense!

Given any set of n numbers, it is ALWAYS possible to find a polynomial of degree (n-1) such that the polynomial generates those numbers.

In this case, try

Un = (19n4 - 270n3 + 1373n2 - 2826n + 2064)/24 for n = 1, 2, 3, 4 etc.