An Improved Di-Directional Heuristic Search Algorithm. - Dennis de Champeaux; Lenie Sint (IJCAI, 1975)

An Optimality Theorem for a Bi-Directional Heuristic Search Algorithm. - Dennis de Champeaux; Lenie Sint (Comput. J., 1977)

An Improved Bidirectional Heuristic Search Algorithm. - Dennis de Champeaux; Lenie Sint (J. ACM, 1977)