18k^2-36k+9 interleaved with 18k^2-18k+9 for k>=0.

Terms

a(0) =9a(1) =9a(2) =-9a(3) =9a(4) =9a(5) =45a(6) =63a(7) =117a(8) =153a(9) =225a(10) =279a(11) =369a(12) =441a(13) =549a(14) =639a(15) =765a(16) =873a(17) =1017a(18) =1143a(19) =1305a(20) =1449a(21) =1629a(22) =1791a(23) =1989a(24) =2169a(25) =2385a(26) =2583a(27) =2817a(28) =3033a(29) =3285