a(n) = A027113(n, 2n-4).

Terms

a(0) =1a(1) =2a(2) =7a(3) =24a(4) =77a(5) =241a(6) =738a(7) =2227a(8) =6650a(9) =19711a(10) =58114a(11) =170682a(12) =499911a(13) =1461284a(14) =4265437a(15) =12438449a(16) =36247956a(17) =105589899a(18) =307513742a(19) =895514715a(20) =2607925982a(21) =7595726764a(22) =22127052759a(23) =64473303788a(24) =187912261301a(25) =547850119836