G.f.: (1+3*x^3)/((1-x)^2*(1-x^3)^2).

Terms

a(0) =1a(1) =2a(2) =3a(3) =9a(4) =15a(5) =21a(6) =36a(7) =51a(8) =66a(9) =94a(10) =122a(11) =150a(12) =195a(13) =240a(14) =285a(15) =351a(16) =417a(17) =483a(18) =574a(19) =665a(20) =756a(21) =876a(22) =996a(23) =1116a(24) =1269a(25) =1422a(26) =1575a(27) =1765a(28) =1955a(29) =2145