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A319489
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Number of non-isomorphic connected graphs on n vertices with representation number 2.
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1
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0, 0, 1, 5, 20, 109, 788, 8335, 117282, 2026330, 40302424, 892278075
(list;
graph;
refs;
listen;
history;
text;
internal format)
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OFFSET
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1,4
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COMMENTS
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These are graphs that can be represented by words having two copies of each letter, but cannot be represented by words having one copy of each letter. In a word representing a graph G, letters x and y alternate if and only if there is an edge between x and y in G. Such graphs, along with complete graphs, are precisely the class of circle graphs.
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LINKS
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EXAMPLE
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For n=3 there is one connected graph with vertex set, say, {1,2}, which is represented by 1212.
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CROSSREFS
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Equals A156808 minus 1; graphs with representation number 3 are in A319490.
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KEYWORD
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nonn,more
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AUTHOR
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STATUS
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approved
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