a(n) = T(2*n-1, n-1), T given by A026519.

Terms

a(0) =1a(1) =2a(2) =8a(3) =28a(4) =111a(5) =436a(6) =1763a(7) =7176a(8) =29521a(9) =122182a(10) =508595a(11) =2126312a(12) =8923136a(13) =37563930a(14) =158563368a(15) =670893296a(16) =2844444761a(17) =12081753410a(18) =51400091942a(19) =218990735668a(20) =934228356445