a(n) = T(2*n, n+1), T given by A027011.

Terms

a(0) =1a(1) =6a(2) =47a(3) =199a(4) =661a(5) =1954a(6) =5442a(7) =14696a(8) =39065a(9) =103025a(10) =270655a(11) =709716a(12) =1859412a(13) =4869594a(14) =12750611a(15) =33383659a(16) =87401977a(17) =228824086a(18) =599072310a(19) =1568395100a(20) =4106115485a(21) =10749954101a(22) =28143749827a(23) =73681298664a(24) =192900149736a(25) =505019154414