a(n) = T(2n+1,n+3), T given by A026758.

Terms

a(0) =1a(1) =7a(2) =37a(3) =177a(4) =809a(5) =3614a(6) =15959a(7) =70101a(8) =307442a(9) =1349374a(10) =5935806a(11) =26195458a(12) =116049130a(13) =516291384a(14) =2307167923a(15) =10357044612a(16) =46705335181a(17) =211564961875a(18) =962540059715