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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

Table of n, a(n) for n=0..19.

Anonymous, Illustration for a(3) = 1

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 A086054

Adjacent sequences:  A078101 A078102 A078103 * A078105 A078106 A078107

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 September 20 22:20 EDT 2019. Contains 327252 sequences. (Running on oeis4.)