a(n) = binomial(2^n - 1, n).A136556a(n) = binomial(2^n - 1, n).Terms a(0) =1a(1) =1a(2) =3a(3) =35a(4) =1365a(5) =169911a(6) =67945521a(7) =89356415775External referencesoeis: A136556