a(n) = T(2n-1,n), where T is the array in A026323.

Terms

a(0) =1a(1) =6a(2) =30a(3) =161a(4) =882a(5) =4916a(6) =27729a(7) =157830a(8) =904706a(9) =5215310a(10) =30203901a(11) =175598997a(12) =1024234850a(13) =5990912280a(14) =35126915550a(15) =206399581434