www.tilepuzzles.com
www.wikipedia.com
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Korf, Richard E. “Recent Progress in the Design and Analysis of Admissible Heuristic Functions.” American Association for Artificial Intelligence, 2000.
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Hansson, Othar and Mayer, Andrew and Yung, Mordechai. “Criticizing Solutions to Relaxed Models Yields Powerful Admissible Heuristics.” Information Sciences: an International Journal. Volume 63, Issue 3. 207-227.
Valtorta, Marco. “A Result on the Computational Complexity of Heuristic Estimates for the A* Algorithm.” Information Science 34, 47-59(1984).
Hansson, Othar and Mayer, Andrew and Valtorta, Marco. “A New Result on The Complexity of Heuristic Estimates for The A* Algorithm.” Artificial Intelligence 55 (1992) 129-143.
Korf’s Slides for “Recent Progress in the Design and Analysis of Admissible Heuristic Functions”.  http://sara2000.unl.edu/Korf-slides.ppt#295,49,Time Complexity of Admissible Heuristic Search Algorithms 


