3 times centered triangular numbers: 9*n*(n+1)/2 + 3.

Terms

a(0) =3a(1) =12a(2) =30a(3) =57a(4) =93a(5) =138a(6) =192a(7) =255a(8) =327a(9) =408a(10) =498a(11) =597a(12) =705a(13) =822a(14) =948a(15) =1083a(16) =1227a(17) =1380a(18) =1542a(19) =1713a(20) =1893a(21) =2082a(22) =2280a(23) =2487a(24) =2703a(25) =2928a(26) =3162a(27) =3405a(28) =3657a(29) =3918