a(n) = 75*(2^n - 1) - 4*n^3 - 18*n^2 - 52*n.

Terms

a(0) =1a(1) =17a(2) =99a(3) =373a(4) =1115a(5) =2901a(6) =6907a(7) =15509a(8) =33483a(9) =70405a(10) =145451a(11) =296997a(12) =601819a(13) =1213493a(14) =2439195a(15) =4893301a(16) =9804587a(17) =19630629a(18) =39286603a(19) =78602885a(20) =157240251a(21) =314520277a(22) =629086139a(23) =1258224213a(24) =2516507275a(25) =5033080901a(26) =10066236267a(27) =20132555749a(28) =40265204123