  Given a positive integer $M$ and a real number $q \in (1,M+1]$, an expansion
of a real number $x \in \left[0,M/(q-1)\right]$ over the alphabet
$A=\{0,1,\ldots,M\}$ is a sequence $(c_i) \in A^{\mathbb N}$ such that
$x=\sum_{i=1}^{\infty}c_iq^{-i}$. Generalizing many earlier results, we
investigate in this paper the topological properties of the set $U_q$
consisting of numbers $x$ having a unique expansion of this form, and the
combinatorial properties of the set $U_q'$ consisting of their corresponding
expansions. We also provide shorter proofs of the main results of Baker in [B]
by adapting the method given in [EJK] for the case $M=1$.
