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Search: id:A109542
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| A109542 |
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a(n) = number of labeled 3-regular (trivalent) multi-graphs without self-loops on 2n vertices with a maximum of 2 edges between any pair of nodes. Also a(n) = number of labeled symmetric 2n X 2n matrices with {0,1,2}-entries with row sum equal to 3 for each row and trace 0. |
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+0
1
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| 0, 7, 640, 170555, 94949400, 95830621425, 159062872168200, 404720953797785625
(list; graph; listen)
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OFFSET
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1,2
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EXAMPLE
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a(2)=7 because for 2*n=4 nodes there are 7 possible labeled graphs whose adjacency matrices are as follows:
0 2 1 0
2 0 0 1
1 0 0 2
0 1 2 0;
0 1 2 0
1 0 0 2
2 0 0 1
0 2 1 0;
0 2 0 1
2 0 1 0
0 1 0 2
1 0 2 0;
0 1 1 1
1 0 1 1
1 1 0 1
1 1 1 0;
0 0 2 1
0 0 1 2
2 1 0 0
1 2 0 0;
0 1 0 2
1 0 2 0
0 2 0 1
2 0 1 0;
0 0 1 2
0 0 2 1
1 2 0 0
2 1 0 0.
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CROSSREFS
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Cf. A001205, A002829, A108243.
Sequence in context: A153405 A087772 A047942 this_sequence A052132 A052134 A101811
Adjacent sequences: A109539 A109540 A109541 this_sequence A109543 A109544 A109545
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KEYWORD
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nonn
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AUTHOR
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Jeremy Gardiner (jeremy.gardiner(AT)btinternet.com), Aug 29 2005
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EXTENSIONS
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a(5)-a(8) from Max Alekseyev (maxale(AT)gmail.com), Aug 30 2005
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