E.g.f. 1/((1-x)(1-x^3)).

Terms

a(0) =1a(1) =1a(2) =2a(3) =12a(4) =48a(5) =240a(6) =2160a(7) =15120a(8) =120960a(9) =1451520a(10) =14515200a(11) =159667200a(12) =2395008000a(13) =31135104000a(14) =435891456000