a(n) = 3 + n/2 + 7*n^2/2.

Terms

a(0) =3a(1) =7a(2) =18a(3) =36a(4) =61a(5) =93a(6) =132a(7) =178a(8) =231a(9) =291a(10) =358a(11) =432a(12) =513a(13) =601a(14) =696a(15) =798a(16) =907a(17) =1023a(18) =1146a(19) =1276a(20) =1413a(21) =1557a(22) =1708a(23) =1866a(24) =2031a(25) =2203a(26) =2382a(27) =2568a(28) =2761a(29) =2961