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A117613
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The number of 3D configurations of a labeled size 2*n "wurfel". A wurfel consists of a series of cubes held together on a looped string such that the string makes a right angle inside each cube. Ignoring the labels would make a wurfel a special kind of polycube in which the cubes can be cyclically ordered at consecutive right angles (equivalently, avoiding 3 consecutive colinear cubes).
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0
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0, 0, 1, 8, 22, 256, 2247, 21576, 225102, 2303014, 24563283, 267169300, 2937239494, 32814269626, 370231763542, 4215902111928
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,4
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COMMENTS
| a(2)=1 since a loop of 2*2=4 labeled cubes has only 1 possible 3D shape. A C program for computing the sequence is available on request.
Richard Gordon from the University of Manitoba (GordonR(AT)ms.umanitoba.ca) first raised the question of how many wurfels there are.
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CROSSREFS
| Sequence in context: A026593 A131622 A183308 * A109271 A029755 A022420
Adjacent sequences: A117610 A117611 A117612 * A117614 A117615 A117616
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KEYWORD
| nonn
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AUTHOR
| John Tromp (tromp(AT)cwi.nl), Apr 06 2006
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EXTENSIONS
| a(15) from John Tromp (tromp(AT)cwi.nl), May 05 2006
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