a(n) = T(2*n,n), T given by A026736.

Terms

a(0) =1a(1) =2a(2) =6a(3) =21a(4) =79a(5) =309a(6) =1237a(7) =5026a(8) =20626a(9) =85242a(10) =354080a(11) =1476368a(12) =6173634a(13) =25873744a(14) =108628550a(15) =456710589a(16) =1922354351a(17) =8098984433a(18) =34147706833a(19) =144068881455a(20) =608151037123