T(2n,n-3), T given by A026780.

Terms

a(0) =1a(1) =15a(2) =144a(3) =1137a(4) =8070a(5) =53678a(6) =342224a(7) =2119359a(8) =12856752a(9) =76826324a(10) =453947206a(11) =2659486160a(12) =15479103062a(13) =89636939038a(14) =517014933254