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A117613 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). 0
0, 0, 1, 8, 22, 256, 2247, 21576, 225102, 2303014, 24563283, 267169300, 2937239494, 32814269626, 370231763542, 4215902111928 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,4

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.

LINKS

Table of n, a(n) for n=0..15.

CROSSREFS

Sequence in context: A131622 A217275 A183308 * A215740 A109271 A029755

Adjacent sequences:  A117610 A117611 A117612 * A117614 A117615 A117616

KEYWORD

nonn

AUTHOR

John Tromp, Apr 06 2006

EXTENSIONS

a(15) from John Tromp, May 05 2006

STATUS

approved

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Last modified February 24 18:24 EST 2018. Contains 299628 sequences. (Running on oeis4.)