Posted by Riana on Tuesday, January 1, 2013 at 1:53pm.
Assuming that's 1, 1, 2, 3, 5... it's the Fibonacci sequence, in which X(i) = X(i-1) + X(i-2), where X(1) = X(2) = 1. So 1+1=2, 1+2=3, 2+3=5, 3+5=8, 5+8=13 etc etc.