A notion of inverse commands is deened for a language with a weakest precondition semantics, permitting both demonic and angelic nondeterminism as well as miracles and nontermination. Every conjunctive and terminating command is invertible, the inverse being non-miraculous and disjunctive. A simulation relation between commands is described using inverse commands. A generalized form of inverse ...

We consider weighted, directed graphs with a notion of absorption on the vertices, related to absorbing random walks on graphs. We define a generalized inverse of the graph Laplacian, called the absorption inverse, that reflects both the graph structure as well as the absorption rates on the vertices. Properties of this generalized inverse are presented, including a matrix forest theorem relati...