a(n) = T(2n,n+2), T given by A026736.

Terms

a(0) =1a(1) =6a(2) =28a(3) =121a(4) =508a(5) =2109a(6) =8723a(7) =36065a(8) =149277a(9) =618961a(10) =2571503a(11) =10704390a(12) =44641793a(13) =186492242a(14) =780275596a(15) =3269135406a(16) =13713525610a(17) =57588530626a(18) =242068874444