n*(1+3*n+6*n^2)/2.

Terms

a(0) =0a(1) =5a(2) =31a(3) =96a(4) =218a(5) =415a(6) =705a(7) =1106a(8) =1636a(9) =2313a(10) =3155a(11) =4180a(12) =5406a(13) =6851a(14) =8533a(15) =10470a(16) =12680a(17) =15181a(18) =17991a(19) =21128a(20) =24610a(21) =28455a(22) =32681a(23) =37306a(24) =42348a(25) =47825a(26) =53755a(27) =60156a(28) =67046a(29) =74443