a(n) = hypergeometric([-n - 1, 1 - n, -n], [1, 3], -1).

Terms

a(0) =1a(1) =1a(2) =3a(3) =12a(4) =55a(5) =276a(6) =1477a(7) =8296a(8) =48393a(9) =291010a(10) =1794320a(11) =11297760a(12) =72413640a(13) =471309944a(14) =3108745785a(15) =20746732688a(16) =139899430981a(17) =952127880138