Coefficient of X^2 in expansion of (1 + n*X + n*X^2)^n.

Terms

a(0) =1a(1) =8a(2) =36a(3) =112a(4) =275a(5) =576a(6) =1078a(7) =1856a(8) =2997a(9) =4600a(10) =6776a(11) =9648a(12) =13351a(13) =18032a(14) =23850a(15) =30976a(16) =39593a(17) =49896a(18) =62092a(19) =76400a(20) =93051a(21) =112288a(22) =134366a(23) =159552a(24) =188125a(25) =220376a(26) =256608a(27) =297136a(28) =342287a(29) =392400