a(n) = 1 + n/2 + 9*n^2/2.

Terms

a(0) =1a(1) =6a(2) =20a(3) =43a(4) =75a(5) =116a(6) =166a(7) =225a(8) =293a(9) =370a(10) =456a(11) =551a(12) =655a(13) =768a(14) =890a(15) =1021a(16) =1161a(17) =1310a(18) =1468a(19) =1635a(20) =1811a(21) =1996a(22) =2190a(23) =2393a(24) =2605a(25) =2826a(26) =3056a(27) =3295a(28) =3543a(29) =3800