a(n) = [x^n] Product_{k>=1} ((1 - x^(3*k))/(1 - x^k))^n.

Terms

a(0) =1a(1) =1a(2) =5a(3) =19a(4) =89a(5) =406a(6) =1913a(7) =9073a(8) =43505a(9) =209971a(10) =1019390a(11) =4971781a(12) =24343037a(13) =119579006a(14) =589050663a(15) =2908727839a(16) =14393759457a(17) =71360342129a(18) =354372852011