Gavin says I must write something. Span wants something mathematical. How about Prime Numbers?
A set of numbers which are not and cannot be Prime.
If a number N, has a whole number square root(r), then clearly it cannot be prime because it has at least three factors, 1*r*r=N.
But did you know that N-1 can't be prime either? Consider the equation (x+1)*(x-1)=M. multiplying out the brackets we get x squared + x -x -1 or M=x squared minus 1. If x is a whole number, then this is the same as making x the same as r in the first equation.
So... r squared =N and r+1 times r-1 = N-1.
There you are Span... maths. Happy?
I'm off work sick today.