a(n) = (1/6)*A290911(n).

Terms

a(0) =0a(1) =1a(2) =4a(3) =16a(4) =68a(5) =287a(6) =1208a(7) =5088a(8) =21432a(9) =90273a(10) =380236a(11) =1601584a(12) =6745996a(13) =28414655a(14) =119684720a(15) =504121280a(16) =2123397744a(17) =8943915201a(18) =37672461204a(19) =158679314512a(20) =668369521108