Introduction
What is discrete differential geometry. A new field of discrete dif-
ferential geometry is presently emerging on the border between differential
and discrete geometry; see, for instance, the recent book Bobenko-Schr¨oder-
Sullivan-Ziegler (2008). Whereas classical differential geometry investigates
smooth geometric shapes (such as surfaces), and discrete geometry studies
geometric shapes with finite number of elements (such as polyhedra), dis-
crete differential geometry aims at the development of discrete equivalents of
notions and methods of smooth surface theory. The latter appears as a limit
of refinement of the discretization. Current interest in this field derives not
only from its importance in pure mathematics but also from its relevance
for other fields: see the lecture course on discrete differential geometry in
computer graphics by Desbrun-Grinspun-Schr¨ oder (2005), the recent book
on architectural geometry by Pottmann-Asperl-Hofer-Kilian (2007), and the
mathematical video on polyhedral meshes and their role in geometry, nu-
merics and computer graphics by Janzen-Polthier (2007).
For a given smooth geometry one can suggest many different discretiza-
tions with the same continuous limit. Which is the best one? From the theo-
retical point of view, one would strive to preserve fundamental properties of
the smooth theory. For applications the requirements of a good discretiza-
tion are different: one aims at the best approximation of a smooth shape,
on the one hand, and at on the other hand, its representation by a discrete
shape with as few elements as possible. Although these criteria are different,
it turns out that intelligent theoretical discretizations are distinguished also
by their good performance in applications. We mention here as an example
the discrete Laplace operator on simplicial surfaces (“cotan formula”) intro-
duced by Pinkall-Polthier (1993) in their investigation of discrete minimal
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