a(n) = binomial(3*n + 1,3).

Terms

a(0) =4a(1) =35a(2) =120a(3) =286a(4) =560a(5) =969a(6) =1540a(7) =2300a(8) =3276a(9) =4495a(10) =5984a(11) =7770a(12) =9880a(13) =12341a(14) =15180a(15) =18424a(16) =22100a(17) =26235a(18) =30856a(19) =35990a(20) =41664a(21) =47905a(22) =54740a(23) =62196a(24) =70300a(25) =79079a(26) =88560a(27) =98770a(28) =109736a(29) =121485