T(2n,n-1), T given by A026703.

Terms

a(0) =1a(1) =6a(2) =31a(3) =155a(4) =767a(5) =3780a(6) =18595a(7) =91393a(8) =448981a(9) =2205125a(10) =10828709a(11) =53172183a(12) =261078623a(13) =1281874806a(14) =6293791663a(15) =30901142675a(16) =151716892649a(17) =744889168771