3 times pentagonal numbers: 3*n*(3*n-1)/2.

Terms

a(0) =0a(1) =3a(2) =15a(3) =36a(4) =66a(5) =105a(6) =153a(7) =210a(8) =276a(9) =351a(10) =435a(11) =528a(12) =630a(13) =741a(14) =861a(15) =990a(16) =1128a(17) =1275a(18) =1431a(19) =1596a(20) =1770a(21) =1953a(22) =2145a(23) =2346a(24) =2556a(25) =2775a(26) =3003a(27) =3240a(28) =3486a(29) =3741