These are some links and papers on bidirectional A* variants. Nothing insightful 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 bidirectional 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.
There 2 papers that cite this one may also be interesting.