Approximate and exact consistency of histories

The consistent histories formalism is discussed using path-projected states. These are used to analyze various criteria for approximate consistency. The connection between the Dowker-Halliwell criterion and sphere packing problems is shown and used to prove several bounds on the violation of probability sum rules. The quantum Zeno effect is also analyzed within the consistent histories formalism and used to demonstrate some of the difficulties involved in discussing approximate consistency. The complications associated with null histories and infinite sets are briefly discussed. © 1996 The American Physical Society.