Numerical Campedelli surfaces are minimal surfaces of general type with vanishing
geometric genus and canonical divisor with self-intersection 2. Although they
have been studied by several authors,their complete classification is not known.
¶ In this paper we classify numerical Campedelli surfaces with an involution,
i.e., an automorphism of order 2. First we show that an involution on a
numerical Campedelli surface $S$ has either four or six isolated fixed points,
and the bicanonical map of $S$ is composed with the involution if and only if
the involution has six isolated fixed points. Then we study in detail each of
the possible cases, describing also several examples.