A078104 - OEIS

A078104

Number of ways a loop can cross three roads meeting in a Y n times. The loop must touch the southwest sector.

1, 0, 2, 1, 7, 6, 37, 42, 237, 320, 1715, 2610, 13478, 22404, 112480, 200158, 982561, 1846314, 8897089, 17481864

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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.

KEYWORD

nonn

AUTHOR

EXTENSIONS

More terms added Aug 25 2003

STATUS

approved