a(n) = (1+n)*(9 + 11*n + 4*n^2)/3.

Terms

a(0) =3a(1) =16a(2) =47a(3) =104a(4) =195a(5) =328a(6) =511a(7) =752a(8) =1059a(9) =1440a(10) =1903a(11) =2456a(12) =3107a(13) =3864a(14) =4735a(15) =5728a(16) =6851a(17) =8112a(18) =9519a(19) =11080a(20) =12803a(21) =14696a(22) =16767a(23) =19024a(24) =21475a(25) =24128a(26) =26991a(27) =30072a(28) =33379a(29) =36920