  Over a field of characteristic zero, it is clear that a polynomial of the
form (X-a)^d has a non-trivial common factor with each of its d-1 first
derivatives. The converse has been conjectured by Casas-Alvero. Up to now there
have only been some computational verifications for small degrees d. In this
paper the conjecture is proved in the case where the degree of the polynomial
is a power of a prime number, or twice such a power.
  Moreover, for each positive characteristic p, we give an example of a
polynomial of degree d which is not a dth power but which has a common factor
with each of its first d-1 derivatives. This shows that the assumption of
characteristic zero is essential for the converse statement to hold.
