a(n) = T(2n,n-2), T given by A026648.

Terms

a(0) =1a(1) =9a(2) =40a(3) =240a(4) =915a(5) =5103a(6) =18704a(7) =101952a(8) =368490a(9) =1990890a(10) =7154928a(11) =38514528a(12) =138086403a(13) =742202055a(14) =2658662560a(15) =14283407616a(16) =51156225342a(17) =274842096462a(18) =984532674800