a(n) = A152800(n+1,2n) for n>=0.

Terms

a(0) =1a(1) =2a(2) =5a(3) =21a(4) =87a(5) =378a(6) =1682a(7) =7596a(8) =34688a(9) =159724a(10) =740243a(11) =3448579a(12) =16134813a(13) =75760294a(14) =356811308a(15) =1684882778a(16) =7974184903a(17) =37815280813a(18) =179644013528a(19) =854749753320