I did consider this, but omitted it earlier since it turned out not to be required. Notice that there is no need to prescribe the shape of the spherical simplex, since some direct Möbius transformation will transform the "octant" into any given proper simplex, while by continuity the number of regions remains constant. For a Euclidean simplex, modify the spherical case by attaching an extra great S^(n-1) , meeting every region except the chosen (closed) simplex and (via symmetry) its antipode. This acts as hyperplane at infinity in an elliptic model of projective space, in which antipodal points map to coincidence. So simplex and antipode map to the same region, leaving 2^(n+1) - 1 regions. WFL On 4/14/16, Dan Asimov <asimov@msri.org> wrote:
Yes — thanks for the correction, Tom:
"...will divide S^n into 2^(n+1) compartments"
is most definitely what I should have written.
—Dan
On Apr 14, 2016, at 1:16 PM, Tom Karzes <karzes@sonic.net> wrote:
Hi Dan,
In the following:
Clearly, doing the same with the right spherical n-simplex (i.e., having all dihedral angles equal to 90º) in the n-sphere S^n will divide S^n into 2^n compartments.
Didn't you mean "...will divide S^n into 2^(n+1) compartments"?
Tom
Dan Asimov writes:
Given a regular (or any) tetrahedron in R^3 with its face planes extended indefinitely will divide R^3 into a certain number of compartments.
A related situation is that of the 2-sphere S^2, where the right spherical triangle (i.e., having all angles equal to 90º) has its 1-faces extended indefinitely to become great circles. Clearly that will divide S^2 into 2^3 = 8 compartments.
Clearly, doing the same with the right spherical n-simplex (i.e., having all dihedral angles equal to 90º) in the n-sphere S^n will divide S^n into 2^n compartments.
NO! This should be 2^(n+1) compartments, as Tom points out.
Puzzle: ------- Given an n-simplex in R^n with its face-hyperplanes extended indefinitely, how many compartments will they divide R^n into ?
—Dan _______________________________________________ math-fun mailing list math-fun@mailman.xmission.com https://mailman.xmission.com/cgi-bin/mailman/listinfo/math-fun
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