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A165642 Number of ways to assemble an n-cube from 2n labeled (n-1)-cubes with labeled vertices, where left-handed and right-handed counterparts are considered distinct. +0
4
2, 96, 7864320, 5917648890101760, 131757106216193173905620336640, 213712134396364787165698675955466240000000000000, 52598387159216969439004693376714880262603303706802782208000000000000000 (list; graph; listen)
OFFSET

1,1

FORMULA

a(n) = 2 * ((2n-2)!!)^(2n-1) * (2n-1)!

EXAMPLE

For n=2, we are constructing a square from 4 labeled line-segments with labeled endpoints. Solutions which differ by a rotation are considered equivalent, but solutions which are a reflection of each other are considered distict (assume the square we are constructing is embedded in a plane, so we cannot flip it over to convert a left-handed solution to right-handed solution). There are 6 ways to order the line-segments, and each line-segment can be oriented in 2 ways, so the total number of solutions is 6 * 2^4 = 96. For n=3, we are constructing a cube from 6 labeled squares with labeled vertices (assume we are confined to 3-space, so we consider reflections of the cube to be distinct). Without loss of generality, we can pick one

labeled square to serve as our face of reference. For this face, we must decide which side of the square will face the interior of the cube, but we do not care about which rotation we pick as these just translate into rotations of the cube. From this reference square, there are 5! ways to assign the remaining squares to the faces of the cube, and each square can be oriented in 8 ways (we can pick which side of the square will face the interior of the cube, and we can pick from 4 rotations). This gives 2 * 8^5 * 5! solutions. The factor of "2" comes from the choice of which side of the reference square will face the interior of the cube (a choice which would go away if we considered reflections to be equivalent).

CROSSREFS

Cf. A165643 (same idea, but reflections are equivalent). A165644 and A091868 are the corresponding sequences for simplices instead of cubes.

Sequence in context: A122222 A036980 A091810 this_sequence A057528 A164335 A132206

Adjacent sequences: A165639 A165640 A165641 this_sequence A165643 A165644 A165645

KEYWORD

nonn

AUTHOR

Andrew Weimholt (andrew(AT)weimholt.com), Sep 23 2009

EXTENSIONS

Example reformatted by Andrew Weimholt (andrew(AT)weimholt.com), Sep 25 2009

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Last modified November 30 13:13 EST 2009. Contains 167758 sequences.


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