  The anomalous properties of the Hall constant in the normal state of
high-$T_c$ superconductors are investigated within the single-band Hubbard
model. We argue that the Mori theory is the appropriate formalism to address
the Hall constant, since it aims directly at resistivities rather than
conductivities. More specifically, the frequency dependent Hall constant
decomposes into its infinite frequency limit and a memory function
contribution. As a first step, both terms are calculated perturbatively in $U$
and on an infinite dimensional lattice, where $U$ is the correlation strength.
If we allow $U$ to be of the order of twice the bare band width, the memory
function contribution causes the Hall constant to change sign as a function of
doping and to decrease as a function of temperature.
