a(n) = 81*n^2 - 9.

Terms

a(0) =72a(1) =315a(2) =720a(3) =1287a(4) =2016a(5) =2907a(6) =3960a(7) =5175a(8) =6552a(9) =8091a(10) =9792a(11) =11655a(12) =13680a(13) =15867a(14) =18216a(15) =20727a(16) =23400a(17) =26235a(18) =29232a(19) =32391a(20) =35712a(21) =39195a(22) =42840a(23) =46647a(24) =50616a(25) =54747a(26) =59040a(27) =63495a(28) =68112a(29) =72891