  Let f(x) be a polynomial of degree at least 5 with complex coefficients and
without repeated roots. Let p be an odd prime. Suppose that all the
coefficients of f(x) lie in a subfield K such that:
  1) K contains a primitive p-th root of unity;
  2) f(x) is irreducible over K;
  3) the Galois group \Gal(f) of f(x) acts doubly transitively on the set of
roots of f(x);
  4) the index of every maximal subgroup of Gal(f) does not divide deg(f)-1.
  Then the endomorphism ring of the Jacobian of the superelliptic curve
y^p=f(x) is isomorphic to the pth cyclotomic ring for all primes p>deg(f).
