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

Terms

a(0) =1a(1) =4a(2) =13a(3) =59a(4) =205a(5) =973a(6) =3479a(7) =16882a(8) =61319a(9) =301486a(10) =1105973a(11) =5485128a(12) =20259303a(13) =101097960a(14) =375265833a(15) =1881279267a(16) =7009601253a(17) =35265787861a(18) =131791097063a(19) =664934132269