a(n) = (2/3)*Sum_{i=1..n-1} A000129(i)*3^(n-i).

Terms

a(0) =0a(1) =0a(2) =2a(3) =10a(4) =40a(5) =144a(6) =490a(7) =1610a(8) =5168a(9) =16320a(10) =50930a(11) =157546a(12) =484120a(13) =1480080a(14) =4507162a(15) =13683050a(16) =41439200a(17) =125259264a(18) =378051170a(19) =1139641930a(20) =3432176008a(21) =10328516880a(22) =31062778570a(23) =93374780426a(24) =280574458640a(25) =842810055360