T(2n,n-1), T given by A027907.

Terms

a(0) =1a(1) =4a(2) =21a(3) =112a(4) =615a(5) =3432a(6) =19383a(7) =110448a(8) =633726a(9) =3656360a(10) =21191555a(11) =123286440a(12) =719539015a(13) =4210967880a(14) =24702429825a(15) =145210795200a(16) =855172338570