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A078104 Number of ways a loop can cross three roads meeting in a Y n times. The loop must touch the southwest sector. 4
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; text; internal format)
OFFSET
0,3
COMMENTS
The Mercedes-Benz problem: closed meanders crossing a Y.
LINKS
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.
CROSSREFS
See A085919 for another version. Cf. A078105 (nonisomorphic solutions), A077460 and A005315 (loop crossing one road).
Cf. also A077550.
Sequence in context: A060583 A246751 A295850 * A072280 A217106 A329995
KEYWORD
nonn
AUTHOR
N. J. A. Sloane and Jon Wild, Dec 05 2002
EXTENSIONS
More terms added Aug 25 2003
STATUS
approved

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Last modified June 23 20:17 EDT 2024. Contains 373653 sequences. (Running on oeis4.)