A* Search

Description:

• Order by the sum of cost in path (as in UCS) and the heuristic (as in Greedy Algorithm)
• Expand the node with the least $f(n)=g(n)+h(n)$
  • cost so far + its heuristics
• A* is complete and optimal, provided that $h(n)$ is admissible for tree search and consistent for graph search

Admissible heuristics:

• Inadmissible heuristic are pessimistic heuristics and break the optimality by trapping good nodes on the Fringe
• A good heuristic should be $0\le h(n)\le h^{\ast }(n)$ where $h^{\ast }(n)$ is the true cost
• However, a admissible heuristic are solutions to related problem
  • where illegal actions are available
  • say the heuristics of going from A to B can be thru a wall

Consistent heuristics:

• $h(s)\le \text{cost}(s\to n)+h(n)$
  • heuristic of a node is less than cost of that node to any of its neighbor and the neighbor’s heuristics
  • think of it as a triangle inequality

    • A*

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• Ensures that the first time exploring any node, it is guaranteed that that node is reached with the shortest path

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