  In this paper, we examine some properties of the fixed point set of a
digitally continuous function. The digital setting requires new methods that
are not analogous to those of classical topological fixed point theory, and we
obtain results that often differ greatly from standard results in classical
topology.
  We introduce several measures related to fixed points for continuous
self-maps on digital images, and study their properties. Perhaps the most
important of these is the fixed point spectrum $F(X)$ of a digital image: that
is, the set of all numbers that can appear as the number of fixed points for
some continuous self-map. We give a complete computation of $F(C_n)$ where
$C_n$ is the digital cycle of $n$ points. For other digital images, we show
that, if $X$ has at least 4 points, then $F(X)$ always contains the numbers 0,
1, 2, 3, and the cardinality of $X$. We give several examples, including $C_n$,
in which $F(X)$ does not equal $\{0,1,\dots,\#X\}$.
  We examine how fixed point sets are affected by rigidity, retraction,
deformation retraction, and the formation of wedges and Cartesian products. We
also study how fixed point sets in digital images can be arranged; e.g., in
some cases the fixed point set is always connected.
