13 votes
Accepted

Why is A* optimal if the heuristic function is admissible?

This is well covered in the corresponding chapter of Russell & Norvig (chapter 3.5, pages 93 to 99 (Third Edition)). Check that out for more details. First, let's review the definitions: Your ...
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5 votes

Is the summation of consistent heuristic functions also consistent?

No, it will not necessary be consistent or admissible. Consider this example, where $s$ is the start, $g$ is the goal, and the distance between them is 1. s --1-- g Assume that $h_0$ and $h_1$ are ...
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  • 351
4 votes
Accepted

Can two admissable heuristics not dominate each other?

This is possible. Admissibility only asserts that the heuristic will never overestimate the true cost. With that being said, it is possible for one heuristic in some cases to do better than another ...
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  • 1,016
2 votes

If an heuristic is not admissible, can it be consistent?

If a heuristic is not admissible, can it be consistent? No. Consistency implies admissibility. In other words, if a heuristic is consistent, it is also admissible. However, admissibility does not ...
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2 votes

If an heuristic is not admissible, can it be consistent?

For a heuristic to be admissible, it must never overestimate the distance from a state to the nearest goal state. For a heuristic to be consistent, the heuristic's value must be less than or equal to ...
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2 votes

How do we determine whether a heuristic function is better than another?

In the A* algorithm, at each iteration, a node is chosen which minimizes a certain function, called the evaluation function, which, in the case of A*, is defined as $$f(n)=g(n)+h(n)$$ where $g(n)$ ...
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2 votes

Understanding the proof that A* search is optimal

The key phrase here is because heuristics are admissible In other words, the heuristics never overestimate the path length: $$cost(n) + heuristic(n) \le cost(\text{any path going through n})$$ And ...
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2 votes
Accepted

Is the minimum and maximum of a set of admissible and consistent heuristics also consistent and admissible?

Yes, in both cases. Below I give two very simple proofs that directly follow from the definitions of admissible and consistent heuristics. However, in a nutshell, the idea of the proofs is that $h_{\...
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2 votes

How do I find whether this heuristic is or not admissible and consistent?

Welcome to AI.SE @hpr16! Your understanding of when a heuristic is admissible is correct, but your heuristic is inadmissible. An admissible heuristic must always underestimate the cost to move from a ...
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1 vote

Is $\min(h_1(s),\ h_2(s))$ consistent?

You can easily find a counterexample. Suppose that there are three nodes $s$, $p$, and $goal$ such that $s \rightarrow p \rightarrow goal$. The real cost of going from $s$ to $p$ is $c(s,p) = 10$ and $...
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  • 1,663
1 vote

If $h_1(n)$ is admissible, why does A* tree search with $h_2(n) = 3h_1(n)$ return a path that is at most thrice as long as the optimal path?

The sketch of the proof for your first question: for an open node $n$, if $f_1(n) = g(n) + h_1(n)$, in the same situation in using $h_2$, it will be $f_2(n) = g(n) + 3 h_1(n)$. Hence, all the time ...
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  • 1,663
1 vote

Is A* with an admissible but inconsistent heuristic optimal?

It depends on what you mean by optimal. A* will always find the optimal solution (that is, the algorithm is admissible) as long as the heuristic is admissible. (Note that the definition of admissible ...
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  • 351
1 vote

If $h_i$ are consistent and admissible, are their sum, maximum, minimum and average also consistent and admissible?

The issue is that you must include assumptions about hopping into your heuristic. In particular, if you are considering individual cars then you must assume that they might be able to hop all of the ...
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  • 351
1 vote
Accepted

Why isn't Nilsson's Sequence Score an admissible heuristic function?

I will use the 8-puzzle game to show you why Nilson's sequence score heuristic function is not admissible. In the 8-puzzle game, you have a $3 \times 3$ board of (numbered) squares as follows. ...
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