a simple basis for the null space of incidence matrix of a graph

In this paper we study the structure of some special bases for the null space of the incidence matrix of a graph. a basis for the null space of the incidence matrix of a graph is called a simple basis, if entries of any vector in this basis is in the set {0,±1}. in this paper we construct a simple basis for any bipartite graph. moreover, we give a necessary and sufficient condition for a graph to have a simple basis.this is based on a joint work with s. akbari, r. khosrovshahi and h. maimani