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We are testing the A* algorithm with Hamming and Manhattan on the 8-puzzle (and its natural generalization n-puzzle) problem. We have to answer the following question but I can't figure out what it should be.

Our assignment is derived from this.

How many board positions are in worst case in memory in function of board size N (where N is the side of a board of size NxN). Give a as low as possible upper bound. You can assume that a board configuration is never in the priority queue several times.

I was thinking about N^2! but that is impossible because not every board position can be reached.

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  • $\begingroup$ @Principis: Websites disappears. When the linked site disappears, your question makes no sense whatsoever anymore. PS. Sam Loyd claimed to have invented it, but he didn't. $\endgroup$ – gnasher729 Apr 29 '18 at 14:08
  • $\begingroup$ @gnasher729 what do you suggest to change it to? The 8-puzzle problem is pretty standard as far as I know. I mean I can't copy the whole assignment here because it's not in English and itś very long. $\endgroup$ – Principis Apr 29 '18 at 15:15
  • $\begingroup$ I've never, ever heard of it under the name. Actually, you could buy cheap plastic toys for what you would call the "15-puzzle" (haven't seen them in years), but I've never heard of either "15-puzzle" nor "8-puzzle". $\endgroup$ – gnasher729 Apr 29 '18 at 17:21
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Since the number of possible positions is quite low when N = 3 (9! / 2 = 181,400), you can do an exhaustive search to find the worst case. Check the result, and maybe it leads to some insight.

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