T(2n,n+3), T given by A026769.

Terms

a(0) =1a(1) =8a(2) =46a(3) =234a(4) =1126a(5) =5280a(6) =24508a(7) =113648a(8) =529555a(9) =2488742a(10) =11824872a(11) =56879504a(12) =277155320a(13) =1368080730a(14) =6838288158a(15) =34588110926a(16) =176875174228a(17) =913577365466