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Search: id:A078105
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| A078105 |
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Number of nonisomorphic ways a loop can cross three roads meeting in a Y n times (orbits under symmetry group of order 6). |
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+0
5
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| 1, 0, 1, 1, 2, 1, 8, 8, 48, 54, 331, 439, 2558, 3734, 21057, 33384, 182293, 307719, 1638465, 2913775, 15181584, 28194412, 144206012, 277887666, 1398566992
(list; graph; listen)
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OFFSET
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0,5
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COMMENT
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There is no constraint on touching any particular sector.
The Mercedes-Benz problem: closed meanders crossing a Y.
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LINKS
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Anonymous, Illustration for a(3) = 1
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EXAMPLE
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With three crossings the loop must cut each road exactly once, so a(3) = 1.
With 4 crossings the loop can cut one road 4 times (one possibility), or two roads twice each (one possibility), so a(4) = 2.
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CROSSREFS
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Cf. A078104 (total number of solutions), A077460 and A005315 (loop crossing one road).
Sequence in context: A016446 A086657 A036296 this_sequence A075513 A011019 A007026
Adjacent sequences: A078102 A078103 A078104 this_sequence A078106 A078107 A078108
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KEYWORD
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nonn,nice
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AUTHOR
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njas and Jon Wild (wild(AT)music.mcgill.ca), Dec 05 2002
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