T(2n,n-1), T given by A026659.

Terms

a(0) =1a(1) =5a(2) =20a(3) =87a(4) =358a(5) =1533a(6) =6445a(7) =27555a(8) =117529a(9) =503001a(10) =2167209a(11) =9289047a(12) =40320051a(13) =173069913a(14) =755481768a(15) =3247042719a(16) =14236984102a(17) =61259521341a(18) =269560841080