a(n) = A127714(n, n*(n+1)/2).

Terms

a(0) =1a(1) =1a(2) =3a(3) =11a(4) =72a(5) =473a(6) =3727a(7) =39646a(8) =412679a(9) =4765654a(10) =61015451a(11) =937386045a(12) =14126033578a(13) =227901084348