Answer for n is n*(n+1)/2*(n+2)

For example if n=2 then answer is 2*3/2*4=12

The formula is correct and is easily proven by solving the sums:

sum(i=0 -> N) sum(j=0 -> N) (i + j)

Edited by author 03.02.2012 17:25

Actually the inner sum needs to start at i, not at zero.   Starting at 0 counts non-double dominoes twice.