Is the summation of consistent heuristic functions also consistent?

3

Imagine that we have a set of heuristic functions $\{h_i\}_{i=1}^N$, where each $h_i$ is both admissible and consistent (monotonic). Is $\sum_{i=1}^N h_i$ still consistent or not?

Is there any proof or counterexample to show the contradiction?

Mostafa Ghadimi

Posted 2020-02-28T15:25:48.877

Reputation: 177

Answers

5

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 perfect heuristics. Then $h_0(s) = 1$ and $h_1(s) = 1$. In this case the heuristic is inadmissible because $h_0(s)+h_1(s) = 2 > d(s, g)$. Similarly, as an undirected graph the heuristic will be inconsistent because $|h(s)-h(g)| > d(s, g)$.

If you'd like to understand the conditions for the sum of heuristics to be consistent and admissible, I would look at the work on additive PDB heuristics.

Nathan S.

Posted 2020-02-28T15:25:48.877

Reputation: 276

Would you please give an example and update your post? – Mostafa Ghadimi – 2020-02-28T16:34:32.407

@MostafaGhadimi In my answer I described an example with two states, the goal state and a second state distance one from the goal. Is there something particular you are looking for more than this? – Nathan S. – 2020-02-28T16:37:42.577

I didn't get what you mean by this example. – Mostafa Ghadimi – 2020-02-28T16:38:17.940

I will add a simple drawing of this example. – Nathan S. – 2020-02-28T16:39:01.310

Thank you very much! – Mostafa Ghadimi – 2020-02-28T16:39:28.480

2For a more extreme version of this answer, consider taking a single admissible, consistent heuristic, and then adding up an infinite number of copies of them. For any base heuristic value $> 0$, this sum is going to end up being $\infty$, which is generally not admissible. – Dennis Soemers – 2020-02-28T16:46:51.143