T(2n,n), T given by A026659.

Terms

a(0) =1a(1) =2a(2) =8a(3) =26a(4) =118a(5) =410a(6) =1946a(7) =6958a(8) =33764a(9) =122638a(10) =602972a(11) =2211946a(12) =10970256a(13) =40518606a(14) =202195920a(15) =750531666a(16) =3762558534a(17) =14019202506a(18) =70531575722a(19) =263582194126