The observability don´t-care set and its approximations

The problem of computing the observability don´t-care set (nonobservability) for a node in a Boolean network is considered. This problem is generally intractable. Recurrence relations are given for two easily computed subsets of the fanout don´t-care set. It is demonstrated that these are, in fact, subsets, and that other subsets appearing in the literature are subsets of those given by the authors