a(n) = (1/3)*A290906(n).

Terms

a(0) =0a(1) =1a(2) =4a(3) =13a(4) =44a(5) =152a(6) =524a(7) =1803a(8) =6204a(9) =21351a(10) =73480a(11) =252880a(12) =870280a(13) =2995049a(14) =10307396a(15) =35472677a(16) =122078436a(17) =420130248a(18) =1445868996a(19) =4975926307a(20) =17124540796a(21) =58933729999a(22) =202819133840a(23) =697997582240