a(n) = (6*n+3)^2.

Terms

a(0) =9a(1) =81a(2) =225a(3) =441a(4) =729a(5) =1089a(6) =1521a(7) =2025a(8) =2601a(9) =3249a(10) =3969a(11) =4761a(12) =5625a(13) =6561a(14) =7569a(15) =8649a(16) =9801a(17) =11025a(18) =12321a(19) =13689a(20) =15129a(21) =16641a(22) =18225a(23) =19881a(24) =21609a(25) =23409a(26) =25281a(27) =27225a(28) =29241a(29) =31329