a(n) = A026626(2*n-1, n-1).

Terms

a(0) =1a(1) =4a(2) =14a(3) =49a(4) =177a(5) =650a(6) =2417a(7) =9071a(8) =34289a(9) =130360a(10) =497928a(11) =1909322a(12) =7345470a(13) =28338826a(14) =109597727a(15) =424761659a(16) =1649314553a(17) =6414825656a(18) =24986917292a(19) =97458970094a(20) =380589237038