T(2n-1,n-1), T given by A026692.

Terms

a(0) =1a(1) =3a(2) =15a(3) =54a(4) =281a(5) =1069a(6) =5682a(7) =22196a(8) =119437a(9) =473871a(10) =2570259a(11) =10302254a(12) =56189736a(13) =226854572a(14) =1242353004a(15) =5042768384a(16) =27702888589a(17) =112915371783a(18) =621846132945