  Let X be a curve in positive characteristic. A Hasse--Witt matrix for X is a
matrix that represents the action of the Frobenius operator on the cohomology
group H^1(X,O_X) with respect to some basis. A Cartier--Manin matrix for X is a
matrix that represents the action of the Cartier operator on the space of
holomorphic differentials of X with respect to some basis. The operators that
these matrices represent are adjoint to one another, so Hasse--Witt matrices
and the Cartier--Manin matrices are related to one another, but there seems to
be a fair amount of confusion in the literature about the exact nature of this
relationship. This confusion arises from differences in terminology, from
differing conventions about whether matrices act on the left or on the right,
and from misunderstandings about the proper formulae for iterating semilinear
operators. Unfortunately, this confusion has led to the publication of
incorrect results. In this paper we present the issues involved as clearly as
we can, and we look through the literature to see where there may be problems.
We encourage future authors to clearly distinguish between Hasse--Witt and
Cartier--Manin matrices, in the hope that further errors can be avoided.
