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A088202
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Chromatic number of the n X n queen graph.
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2
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1, 4, 5, 5, 5, 7, 7, 9, 10, 11, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26
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OFFSET
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1,2
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COMMENTS
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a(27) is the first open case.
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REFERENCES
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M. Gardner, The Unexpected Hanging and Other Mathematical Diversions. Simon and Schuster, NY, 1969, p. 191.
Thorold Gosset, Mess. Math., 44 (1914), 48. (Shows a(8) > 8.)
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LINKS
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Witold Jarnicki, W. Myrvold, P. Saltzman, S. Wagon, Properties, Proved and Conjectured, of Keller, Mycielski, and Queen Graphs, arXiv preprint arXiv:1606.07918 [math.CO], 2016.
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FORMULA
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Sequence is monotonic and a(n) = n if n is a prime > 3.
a(n) = n if n == 1 or 5 (mod 6).
a(n) <= p := nextprime(n), since we can simply take a solution for p and remove the last n-p rows and columns.
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EXAMPLE
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A 10-coloring of the 9 X 9 chessboard, showing that a(9) <= 10:
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0 2 1 7 3 9 5 8 6
1 3 4 5 0 8 6 9 2
2 0 6 8 4 3 1 5 7
3 1 7 9 5 2 4 6 0
4 6 3 2 7 0 8 1 9
5 7 9 4 6 1 3 0 8
6 4 0 1 9 5 2 7 3
7 5 8 3 2 6 9 4 1
8 9 2 6 1 4 0 3 5
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CROSSREFS
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KEYWORD
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nonn,more,hard
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AUTHOR
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Willem Haemers (Haemers(AT)uvt.nl), Nov 03 2003
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EXTENSIONS
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Entry revised Mar 22 2004 using material from the Chvatal web site.
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STATUS
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approved
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