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Determine or estimate the number of maximal triangle-free graphs on $n$ vertices
Among the collections of the open problems of Paul Erdős on the website of
Professor Fan Chung, there is one called "number of triangle-free graphs".
http://www.math.ucsd.edu/~erdosproblems/erdos/...
52
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4
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Do there exist chess positions that require exponentially many moves to reach?
By "chess" here I mean chess played on an $n\times n$ board with an unbounded number of (non-king) pieces. Some care is needed if you want to generalize some of the subtler rules of chess to an $n\...