Bidirectional A*

These are some links and papers on bidirectional A* variants. Nothing insightfull at the moment.

small lecture
introduces \(h_f(u) = \frac 12 (\pi_f(u) - \pi_r)\). Not found a paper yet.
An Improved Bidirectional Heuristic Search Algorithm (Champeaux 1977)
introduces a bidirectoinal variant
Bidirectional Heuristic Search Again (Champeaux 1983)
fixes a bug in the above paper
Efficient modified bidirectional A* algorithm for optimal route-finding
Didn’t read closely yet.
A new bidirectional algorithm for shortest paths (Pijls 2008)
Actually a new methods. Seems to cite useful papers.