a(n) = n*(7*n - 1)/2.

Terms

a(0) =0a(1) =3a(2) =13a(3) =30a(4) =54a(5) =85a(6) =123a(7) =168a(8) =220a(9) =279a(10) =345a(11) =418a(12) =498a(13) =585a(14) =679a(15) =780a(16) =888a(17) =1003a(18) =1125a(19) =1254a(20) =1390a(21) =1533a(22) =1683a(23) =1840a(24) =2004a(25) =2175a(26) =2353a(27) =2538a(28) =2730a(29) =2929