a(n) = T(2*n, n-2), where T is given by A026552.

Terms

a(0) =1a(1) =4a(2) =18a(3) =74a(4) =311a(5) =1296a(6) =5432a(7) =22796a(8) =95958a(9) =404812a(10) =1711600a(11) =7250970a(12) =30772989a(13) =130810512a(14) =556867224a(15) =2373764416a(16) =10130935783a(17) =43285462884a(18) =185129287262a(19) =792525473552