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A078104
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Number of ways a loop can cross three roads meeting in a Y n times. The loop must touch the South-West sector.
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4
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1, 0, 2, 1, 7, 6, 37, 42, 237, 320, 1715, 2610, 13478, 22404, 112480, 200158, 982561, 1846314, 8897089, 17481864
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,3
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COMMENTS
| The Mercedes-Benz problem: closed meanders crossing a Y.
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LINKS
| Anonymous, Illustration for a(3) = 1
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EXAMPLE
| 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 (giving A005315(2)*2 = 4 possibilities), or two roads twice each (3 ways), so a(4) = 7.
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CROSSREFS
| See A085919 for another version. Cf. A078105 (nonisomorphic solutions), A077460 and A005315 (loop crossing one road).
Cf. also A077550.
Sequence in context: A204912 A178049 A060583 * A072280 A086054 A011134
Adjacent sequences: A078101 A078102 A078103 * A078105 A078106 A078107
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KEYWORD
| nonn
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AUTHOR
| N. J. A. Sloane (njas(AT)research.att.com) and Jon Wild (wild(AT)music.mcgill.ca), Dec 05 2002
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EXTENSIONS
| More terms added Aug 25, 2003
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