T(2n,n-2), T given by A026568.

Terms

a(0) =1a(1) =3a(2) =18a(3) =65a(4) =346a(5) =1288a(6) =6818a(7) =25518a(8) =136152a(9) =510532a(10) =2745347a(11) =10309234a(12) =55784794a(13) =209766714a(14) =1140670242a(15) =4294635438a(16) =23445204441