a(n) = 1 - 2*n^2 + 4*n*(1 + 2*n^2)/3.

Terms

a(0) =1a(1) =3a(2) =17a(3) =59a(4) =145a(5) =291a(6) =513a(7) =827a(8) =1249a(9) =1795a(10) =2481a(11) =3323a(12) =4337a(13) =5539a(14) =6945a(15) =8571a(16) =10433a(17) =12547a(18) =14929a(19) =17595a(20) =20561a(21) =23843a(22) =27457a(23) =31419a(24) =35745a(25) =40451a(26) =45553a(27) =51067a(28) =57009a(29) =63395