  The resonant behaviour of vacancy states in graphene is well-known but some
ambiguities remain concerning in particular the nature of the so-called zero
energy modes. Other points are not completely elucidated in the case of low but
finite vacancy concentration. In this article we concentrate on the case of
vacancies described within the usual tight-binding approximation. More
precisely we discuss the case of a single vacancy or of a finite number of
vacancies in a finite or infinite system.
