a(n) = (1/3)*A291232(n).

Terms

a(0) =2a(1) =9a(2) =38a(3) =153a(4) =596a(5) =2268a(6) =8480a(7) =31275a(8) =114086a(9) =412443a(10) =1479926a(11) =5276664a(12) =18711758a(13) =66041901a(14) =232129190a(15) =812934621a(16) =2837740232a(17) =9877082004a(18) =34288573484a(19) =118752490863a(20) =410394698534