a(n) = Sum_{i=0..n} binomial(2*n, i).

Terms

a(0) =1a(1) =3a(2) =11a(3) =42a(4) =163a(5) =638a(6) =2510a(7) =9908a(8) =39203a(9) =155382a(10) =616666a(11) =2449868a(12) =9740686a(13) =38754732a(14) =154276028a(15) =614429672a(16) =2448023843a(17) =9756737702a(18) =38897306018a(19) =155111585372a(20) =618679078298