Binomial coefficients C(2n+1,n) repeated.

Terms

a(0) =1a(1) =1a(2) =3a(3) =3a(4) =10a(5) =10a(6) =35a(7) =35a(8) =126a(9) =126a(10) =462a(11) =462a(12) =1716a(13) =1716a(14) =6435a(15) =6435a(16) =24310a(17) =24310a(18) =92378a(19) =92378a(20) =352716a(21) =352716a(22) =1352078a(23) =1352078a(24) =5200300a(25) =5200300a(26) =20058300a(27) =20058300a(28) =77558760a(29) =77558760