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 A078675 Number of ways to lace a shoe that has n pairs of eyelets. 3
 1, 2, 14, 322, 17314, 1573952, 210985926, 38916737688 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS The lace must pass through each eyelet exactly once, must begin and end at the extreme pair of eyelets and cannot pass in order though three adjacent eyelets that are in a line. The lace is "undirected": reversing the order of eyelets along the path does not count as a different solution. LINKS N. J. A. Sloane, FORTRAN program EXAMPLE a(3) = 14: label the eyelets 1,2,3 from front to back on the left side then 4,5,6 from back to front on the right side. The lacings are: 124356 154326 153426 142536 145236 132546 135246 125346 124536 125436 152346 153246 152436 154236. CROSSREFS Cf. A078602 for directed solutions, A078676 for symmetric solutions. a(n) = ( A078602(n) + A078676(n) ) / 2 Sequence in context: A128087 A139225 A277035 * A133130 A165696 A180605 Adjacent sequences:  A078672 A078673 A078674 * A078676 A078677 A078678 KEYWORD nonn AUTHOR N. J. A. Sloane, Dec 11 2002 EXTENSIONS a(7) and a(8) from Hugo Pfoertner, Jan 22 2005 STATUS approved

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Last modified July 25 20:54 EDT 2021. Contains 346291 sequences. (Running on oeis4.)