Morse functions and orbifolds are separate concepts used in global analysis and geometric topology, respectively. We find that a combination of the two is useful for classification purposes in the unofficial crystallographic orbifold atlas we are compiling.
Using peak-pass-pale-pit terminology for the four types of non-degenerate critical points (zero gradient points) in 3-Euclidean space, a "Crystallographic Morse Function" (CMF) is an infinite pattern of 3-D isotropic normal probability density functions (pdf) centered on special space group orbit (Wyckoff) positions. The peak positions are at the pdf centers and the overlapping pdf tails determine the pass, pale, pit, and gradient-flow separatrices arrangement. The arrangement is then "orbi-folded" to form a 3-Euclidean orbifold with Morse function superimposed.
The model utilizes and extends the crystallographic lattice-complex concept (defined in Chapter 14 of the "International Tables for Crystallography, Vol. A (1995)" as "the set of all point configurations that may be generated within one type of Wyckoff set (orbit)"). For example, the basic CMF orbifold in space group Im3m is the body centered cubic arrangement that contains eight lattice complexes, i.e., four 0-D critical points, three 1-D topologically unique gradient-flow separatrices joining consecutive critical points, and one shortest path between the peak and pit. Each of these elements is contained within a single 2-elliptic orbifold domain within the 3-Euclidean orbifold. A node-link directed graph, representing this configuration and the associated Wyckoff multiplicities, characterizes both the CMF and the space-group orbifold. The Morse function provides a unique format for drawing and comparing orbifold graphs because of the Morse mapping onto a 1-D Euclidean space. A "duplex" matroid representation also exists with the elliptic 2-orbifold singular-set arrangement in one direction and Morse CW-complex poset arrangement in the other.
We conjecture that in conjunction with our previously described "color orbifold covers" specification of group / normal subgroup pairs of orbifolds based on color groups, the crystallographic 3-Euclidean orbifolds should be classified and documented using the embedding characteristics for some basis set of CMFs. For crystal chemistry research, this would provide a very useful and geometrically intuitive classification. We are currently working with the cubic orbifolds.
Critiques on this approach and pointers to related literature and research combining orbifolds and Morse theory would be appreciated.
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