a(n) = n*(2*n+5)*(n-1)/6.

Terms

a(0) =0a(1) =0a(2) =3a(3) =11a(4) =26a(5) =50a(6) =85a(7) =133a(8) =196a(9) =276a(10) =375a(11) =495a(12) =638a(13) =806a(14) =1001a(15) =1225a(16) =1480a(17) =1768a(18) =2091a(19) =2451a(20) =2850a(21) =3290a(22) =3773a(23) =4301a(24) =4876a(25) =5500a(26) =6175a(27) =6903a(28) =7686a(29) =8526