a(n) = T(2n,n), where T is the array in A026148.

Terms

a(0) =1a(1) =1a(2) =7a(3) =35a(4) =189a(5) =1038a(6) =5796a(7) =32734a(8) =186494a(9) =1069793a(10) =6170554a(11) =35752906a(12) =207940493a(13) =1213271620a(14) =7098581115a(15) =41631465591a(16) =244669507994