a(n) = Sum_{d|n} binomial(2*d-1,d).

Terms

a(0) =1a(1) =4a(2) =11a(3) =39a(4) =127a(5) =476a(6) =1717a(7) =6474a(8) =24321a(9) =92508a(10) =352717a(11) =1352589a(12) =5200301a(13) =20060020a(14) =77558897a(15) =300546669a(16) =1166803111a(17) =4537592436a(18) =17672631901a(19) =68923356953a(20) =269128938947