Sum{T(n,k)*T(n,2n-k)}, 0<=k<=n, T given by A027926.

Terms

a(0) =1a(1) =2a(2) =7a(3) =21a(4) =65a(5) =194a(6) =573a(7) =1668a(8) =4809a(9) =13745a(10) =39011a(11) =110052a(12) =308857a(13) =862886a(14) =2401195a(15) =6658509a(16) =18406373a(17) =50738918a(18) =139513269a(19) =382731240a(20) =1047768621a(21) =2862908777a(22) =7808872007a(23) =21265115976a(24) =57822787825a(25) =157010774474a(26) =425795804623