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A001997
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Different shapes formed by bending a piece of wire of length n in the plane.
(Formerly M1206 N0465)
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8
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1, 1, 2, 4, 10, 24, 66, 176, 493, 1362, 3821, 10660, 29864, 83329, 232702, 648182, 1804901, 5015725, 13931755, 38635673, 107090666, 296449133, 820271143, 2267225157, 6264244414, 17291930470
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,3
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COMMENTS
| Wire is marked into n equal segments by n-1 marks, is bent at right angles at each of one or more of these points, making each segment parallel to one of two rectangular axes. (Stays in plane, bends are of 0 or +-90 degs.) May cross itself but is not self-coincident over a finite length.
A trail is a path which may cross itself but does not reuse an edge. This sequence counts undirected trails on the square lattice up to rotation and reflection. Directed trails are counted by A006817.
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REFERENCES
| R. M. Foster, Solution to Problem E185, Amer. Math. Monthly, 44 (1937), 50-51.
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
| Eric Weisstein's World of Mathematics, Link to a section of The World of Mathematics.
Index entries for sequences obtained by enumerating foldings
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EXAMPLE
| ._. ._._._. Here are the
|_. . ._. . 4 solutions
._._| . |_. when n=3 (described by 00, RR, 0L, RL).
The 24 solutions for n=5 are 0000, 000R, 00R0, 00RR, 00RL, L00L, L00R, 0R0R, 0R0L, 0RR0, 0RL0, 0LRL, 0LRR, 0LLR, 0LLL, R0LR, R0LL, R0RL, R0RR, LRLR, LRLL, LRLR, LRRR, LLRR.
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CROSSREFS
| Total number of different shapes (including those shapes where the wire is self-coincident over a finite path) is A001998.
Sequence in context: A137842 A049146 A000682 * A000084 A057734 A151516
Adjacent sequences: A001994 A001995 A001996 * A001998 A001999 A002000
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KEYWORD
| nonn,more,nice,walk
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AUTHOR
| N. J. A. Sloane (njas(AT)research.att.com).
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EXTENSIONS
| More terms from David W. Wilson (davidwwilson(AT)comcast.net), Jul 18 2001
Much less is known about the three-dimensional problem.
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