The article surveys the known results and conjectures about the analytic properties of dispersion relations and Fermi surfaces for periodic equations of mathematical physics and their spectral incarnations.

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97.
The specific choice of a co-compact abelian sub-group of shifts is irrelevant at this, and many others, moment.
98.
Needed, for instance, to allow for presence of constant magnetic field. The corresponding magnetic translations group of Zak transformations is non-abelian (see, e.g., Refs. 2 and 91).
99.
Infinite differentiability of the coefficients is a significant overkill, but we will not address the issues of minimal conditions on the coefficients.
100.
One can handle to some extent the hypo-elliptic, e.g., parabolic equations, while things get much harder there.50 
101.
This labeling disappears when k is complex.
102.
It is saying that a solution vanishing in an open set is identically zero.
103.
It is interesting to compare the spider decoration with the zig-zag product used to produce expander graphs.77 
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