a(n) = T(2n,n-1), where T is the array in A026323.

Terms

a(0) =1a(1) =4a(2) =21a(3) =112a(4) =615a(5) =3432a(6) =19382a(7) =110432a(8) =633555a(9) =3654840a(10) =21179389a(11) =123195360a(12) =718887585a(13) =4206458700a(14) =24671967210a(15) =145008786304a(16) =853852037038