a(n) = (6*n^2 + 7*n - 9 + 2*n^3)/12 - (-1)^n*(n+1)/4.

Terms

a(0) =1a(1) =3a(2) =11a(3) =19a(4) =37a(5) =55a(6) =87a(7) =119a(8) =169a(9) =219a(10) =291a(11) =363a(12) =461a(13) =559a(14) =687a(15) =815a(16) =977a(17) =1139a(18) =1339a(19) =1539a(20) =1781a(21) =2023a(22) =2311a(23) =2599a(24) =2937a(25) =3275a(26) =3667a(27) =4059a(28) =4509a(29) =4959