Size functor
Given a size pair where
is a manifold of dimension
and
is an arbitrary real continuous function defined
on it, the
-th size functor,[1] with
, denoted
by
, is the functor in
, where
is the category of ordered real numbers, and
is the category of Abelian groups, defined in the following way. For
, setting
,
,
equal to the inclusion from
into
, and
equal to the morphism in
from
to
,
- for each
,
-
In other words, the size functor studies the
process of the birth and death of homology classes as the lower level set changes.
When is smooth and compact and
is a Morse function, the functor
can be
described by oriented trees, called
− trees.
The concept of size functor was introduced as an extension to homology theory and category theory of the idea of size function. The main motivation for introducing the size functor originated by the observation that the size function can be seen as the rank
of the image of
.
The concept of size functor is strictly related to the concept of persistent homology group
,[2]
studied in persistent homology. It is worth to point out that the -th persistent homology group coincides with the image of the homomorphism
.
References
- ↑ Francesca Cagliari, Massimo Ferri, Paola Pozzi, Size functions from a categorical viewpoint, Acta Applicandae Mathematicae, 67(3):225-235, 2001.
- ↑ Herbert Edelsbrunner, David Letscher, Afra Zomorodian, Topological Persistence and Simplification, Discrete and Computational Geometry, 28(4):511-533, 2002.