a(n) = 1*binomial(n,2) + 3*binomial(n,3) + 6*binomial(n,4) + 10*binomial(n,5).

Terms

a(0) =0a(1) =0a(2) =1a(3) =6a(4) =24a(5) =80a(6) =225a(7) =546a(8) =1176a(9) =2304a(10) =4185a(11) =7150a(12) =11616a(13) =18096a(14) =27209a(15) =39690a(16) =56400a(17) =78336a(18) =106641a(19) =142614a(20) =187720a(21) =243600a(22) =312081a(23) =395186a(24) =495144a(25) =614400a(26) =755625a(27) =921726a(28) =1115856a(29) =1341424