a(n) = T(3n+1,n), where T = Catalan triangle (A008315).

A026004

a(n) = T(3n+1,n), where T = Catalan triangle (A008315).

Terms

    a(0) =1a(1) =3a(2) =14a(3) =75a(4) =429a(5) =2548a(6) =15504a(7) =95931a(8) =600875a(9) =3798795a(10) =24192090a(11) =154969620a(12) =997490844a(13) =6446369400a(14) =41802112192a(15) =271861216539

External references