On graceful difference labelings of disjoint unions of circuits

A graceful difference labeling (gdl for short) of a directed graph G with vertex set V is a bijection f : V → {1,. .. , |V|} such that, when each arc uv is assigned the difference label f (v) − f (u), the resulting arc labels are distinct. We conjecture that all disjoint unions of circuits have a gdl, except in two particular cases. We prove partial results which support this conjecture.