a(n) = A026681(2n,n).

Terms

a(0) =1a(1) =3a(2) =10a(3) =44a(4) =174a(5) =822a(6) =3414a(7) =16604a(8) =70600a(9) =348828a(10) =1503802a(11) =7504156a(12) =32645122a(13) =164014374a(14) =718098056a(15) =3625748104a(16) =15950473396a(17) =80839381968a(18) =356946436186