a(n) = T(2n,n), where T is the array in A026268.

Terms

a(0) =1a(1) =1a(2) =5a(3) =27a(4) =145a(5) =796a(6) =4438a(7) =25034a(8) =142479a(9) =816620a(10) =4706977a(11) =27256963a(12) =158450461a(13) =924129535a(14) =5404960900a(15) =31689099660a(16) =186188338689