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 A079410 Number of ways to lace a shoe that has n pairs of eyelets such that the lace does not cross itself between the eyelet rows. 0
 2, 7, 54, 313, 2890, 25764 (list; graph; refs; listen; history; text; internal format)
 OFFSET 3,1 COMMENTS The lace must pass through each eyelet exactly once, must begin and end at the extreme pair of eyelets and each eyelet must have at least one direct connection to the opposite side. The corresponding sequence including all configs where the lace crosses itself in the space between the eyelet rows is A078698. The only symmetric crossing-free lacing is 1234 for N=2. LINKS Hugo Pfoertner, FORTRAN program, illustration of lacings for N=3,4 EXAMPLE With the notation introduced in A078602, the 4 crossing-free lacings for N=3 are 125346, 134256, 134526, 152346. Not counting mirror images we get a(3)=2. Lists of all crossing-free lacings for N=3,4,5,6 and illustrations of the lacings can be found following the FORTRAN program at the Pfoertner link. PROG FORTRAN program provided at Pfoertner link (including a subroutine LPG for lexicographic permutation generation) CROSSREFS Cf. A078602, A078698, A000384 (the maximum number of lace crossings that can occur in an n-eyelet pair shoe lacing is A000384(n-1)). Sequence in context: A180720 A168558 A024027 * A283335 A280221 A227381 Adjacent sequences:  A079407 A079408 A079409 * A079411 A079412 A079413 KEYWORD nonn AUTHOR Hugo Pfoertner, Jan 06 2003 STATUS approved

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