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A078105 Number of nonisomorphic ways a loop can cross three roads meeting in a Y n times (orbits under symmetry group of order 6). 5
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; refs; listen; history; text; internal format)
OFFSET

0,5

COMMENTS

There is no constraint on touching any particular sector.

The Mercedes-Benz problem: closed meanders crossing a Y.

LINKS

Table of n, a(n) for n=0..24.

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 (one possibility), or two roads twice each (one possibility), so a(4) = 2.

CROSSREFS

Cf. A078104 (total number of solutions), A077460 and A005315 (loop crossing one road).

Sequence in context: A086657 A188922 A036296 * A075513 A284211 A246403

Adjacent sequences:  A078102 A078103 A078104 * A078106 A078107 A078108

KEYWORD

nonn,nice

AUTHOR

N. J. A. Sloane and Jon Wild, Dec 05 2002

STATUS

approved

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Last modified September 17 08:57 EDT 2019. Contains 327128 sequences. (Running on oeis4.)