For a given positive integer y, sequence c is defined this way:
  
  c[2] = frac(y/2)
  c[n+1] = frac(c[n]-y/(n^2+n))

The sequence is defined for n = <2,int(y/2)-1>.

Devise a formula or an algorithm to find values of y for which
no element of c is zero. Must be O(log10(y)) or better.

Mail answers to lcamtuf@coredump.cx. Modest prizes available.