Re: [math-fun] How many pieces? puzzle
That sounds like the checkerboard lattice D_n in n-space, D_n = {(x_1, ..., x_n) in Z^n | x_1 + ... + x_n == 0 mod 2} —Dan ----- the unit-sphere packing where centres have evenly many odd integer Cartesian components. -----
Errrm. Either I need to think a bit more about firming my argument up, or I have a touch of incipient MFA syndrome ... Watch this space, folks. WFL On 9/21/18, Dan Asimov <dasimov@earthlink.net> wrote:
That sounds like the checkerboard lattice D_n in n-space,
D_n = {(x_1, ..., x_n) in Z^n | x_1 + ... + x_n == 0 mod 2}
—Dan
----- the unit-sphere packing where centres have evenly many odd integer Cartesian components. -----
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certainly the n-dim lattice "integer coords with even sum" is the D_n lattice. In 2-D it is the checkerboard in 3-D it is the face-centered cubic lattice there are two tilings that go with it, the main one being the decomposition of space into the Voronoi cells around the lattice points the other one is the dual tiling, into Delaunay regions, which are centered at the holes in the lattice (meaning the points that are locally maximally distant from the lattice points). Best regards Neil Neil J. A. Sloane, President, OEIS Foundation. 11 South Adelaide Avenue, Highland Park, NJ 08904, USA. Also Visiting Scientist, Math. Dept., Rutgers University, Piscataway, NJ. Phone: 732 828 6098; home page: http://NeilSloane.com Email: njasloane@gmail.com On Fri, Sep 21, 2018 at 10:00 AM, Fred Lunnon <fred.lunnon@gmail.com> wrote:
Errrm. Either I need to think a bit more about firming my argument up, or I have a touch of incipient MFA syndrome ...
Watch this space, folks.
WFL
On 9/21/18, Dan Asimov <dasimov@earthlink.net> wrote:
That sounds like the checkerboard lattice D_n in n-space,
D_n = {(x_1, ..., x_n) in Z^n | x_1 + ... + x_n == 0 mod 2}
—Dan
----- the unit-sphere packing where centres have evenly many odd integer Cartesian components. -----
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As Dan observed, the set of points I was clumsily attempting to define is simply the "checkerboard" lattice D_n = { (x_1, ..., x_n) in |Z^n | x_1 + ... + x_n == 0 mod 2 } , which is congruent to its complementary D'_n = { (x_1, ..., x_n) in |Z^n | x_1 + ... + x_n == 1 mod 2 } . Now define 2^(n-1) pencils of parallel hyperplanes x_1 +/- ... +/- x_n = 2 k , one for each possible choice of signs: these hyperplanes bisect vertex diagonals of lattice hypercubes odd vertices, satisfying x_1, ..., x_n == 1 mod 2 . [ Notice that hypercube centres reside always in D_n , while the vertices reside in D_n , D'_n for n even, odd respectively. For example when n = 3 , bisector planes meet a cube in hexagons with vertices at mid-points of cube edges, which together with cube centres constitute D_3 ; while cube vertices and face centres constitute D'_3 . ] Each point of D_n lies on a unique member of every pencil: the combined configuration constitutes the universal cover of Dan's partitioned n-tore; its points are the vertices of "piece" polytopes, whose facets tile the hyperplanes. The piece tessellation must just be the Delaunay triangulation Del(D_n) mentioned by Neil. Quotienting out the even lattice yields a periodic n-tore with content 2^n and 2^(n-1) distinct congruent vertices; we require to enumerate the types and frequencies of pieces occurring per period. My previous assertion concerning these was over-impetuous: it seems safe to assert that one type comprises 2^(n-1) regular cross-polytopes centred on points of D'_n ; however occupancy of the voids in between remains obscure to me for n > 4 . Case n = 3 : pieces comprise 4 octahedra and 8 tetrahedra, vertices 4 cuboctahedra, from tessellation h{4,3,4} : see https://en.wikipedia.org/wiki/Tetrahedral-octahedral_honeycomb Case n = 4 : pieces comprise 24 cross-polytopes (dual tesseracts), vertices 8 icositetratopes, from tessellation {3,3,4,3} : see https://en.wikipedia.org/wiki/24-cell_honeycomb This topic must surely have already been explored by others --- Conway & Sloane "Sphere Packings, Lattices, and Groups"; Coxeter "Regular Polytopes"; or perhaps the Rostock group involved in "Delaunay Tessellations of Point Lattices" (ERC Workshop Oct 2013) http://www.mi.fu-berlin.de/en/math/groups/discgeom/dates/2nd_ERC_Workshop/sl... Fred Lunnon
participants (3)
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Dan Asimov -
Fred Lunnon -
Neil Sloane