a(n) = A026626(2*n, n).

Terms

a(0) =1a(1) =3a(2) =8a(3) =28a(4) =98a(5) =354a(6) =1300a(7) =4834a(8) =18142a(9) =68578a(10) =260720a(11) =995856a(12) =3818644a(13) =14690940a(14) =56677652a(15) =219195454a(16) =849523318a(17) =3298629106a(18) =12829651312a(19) =49973834584a(20) =194917940188a(21) =761178474076