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

Terms

a(0) =3a(1) =26a(2) =85a(3) =196a(4) =375a(5) =638a(6) =1001a(7) =1480a(8) =2091a(9) =2850a(10) =3773a(11) =4876a(12) =6175a(13) =7686a(14) =9425a(15) =11408a(16) =13651a(17) =16170a(18) =18981a(19) =22100a(20) =25543a(21) =29326a(22) =33465a(23) =37976a(24) =42875a(25) =48178a(26) =53901a(27) =60060a(28) =66671a(29) =73750