a(n) = T(2*n, n), T given by A027907.

Terms

a(0) =1a(1) =2a(2) =10a(3) =50a(4) =266a(5) =1452a(6) =8074a(7) =45474a(8) =258570a(9) =1481108a(10) =8533660a(11) =49402850a(12) =287134346a(13) =1674425300a(14) =9792273690a(15) =57407789550a(16) =337281021450