T(2n,n-1), T given by A026681.

Terms

a(0) =1a(1) =7a(2) =26a(3) =150a(4) =561a(5) =3181a(6) =12101a(7) =68514a(8) =263959a(9) =1496375a(10) =5818228a(11) =33039504a(12) =129350028a(13) =735726968a(14) =2895663570a(15) =16493980890a(16) =65189381839a(17) =371790856907