a(n) = (-1)^(n+1)*Sum_{k=0..n+1}(-1)^k*binomial(2*k,k).

Terms

a(0) =1a(1) =5a(2) =15a(3) =55a(4) =197a(5) =727a(6) =2705a(7) =10165a(8) =38455a(9) =146301a(10) =559131a(11) =2145025a(12) =8255575a(13) =31861025a(14) =123256495a(15) =477823895a(16) =1855782325a(17) =7219352975a(18) =28125910825a(19) =109720617995a(20) =428537256445