a(n) = A026637(2*n, n).

Terms

a(0) =1a(1) =2a(2) =8a(3) =26a(4) =92a(5) =332a(6) =1220a(7) =4538a(8) =17036a(9) =64412a(10) =244928a(11) =935684a(12) =3588392a(13) =13806704a(14) =53271548a(15) =206040506a(16) =798600332a(17) =3101109164a(18) =12062148368a(19) =46986821516a(20) =183276382472a(21) =715748620424