you have given N. you need to find out C(n,1*1)+C(n,2*2)+C(n,3*3)+c(n,4*4) + .....
Here c(n,r) = n!/((n-r)!*(r)!)
one way is find C(n,i*i) for all i between 1 <= i <= sqrt(n)
. Is there exist any efficient solution than this ??