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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 June 22 11:56 EDT 2024. Contains 373570 sequences. (Running on oeis4.)