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/* This file is part of the Gudhi Library - https://gudhi.inria.fr/ - which is released under MIT.
* See file LICENSE or go to https://gudhi.inria.fr/licensing/ for full license details.
* Author(s): Siddharth Pritam
*
* Copyright (C) 2020 Inria
*
* Modification(s):
* - YYYY/MM Author: Description of the modification
*/
#ifndef DOC_EDGE_COLLAPSE_INTRO_EDGE_COLLAPSE_H_
#define DOC_EDGE_COLLAPSE_INTRO_EDGE_COLLAPSE_H_
namespace Gudhi {
namespace collapse {
/** \defgroup edge_collapse Edge collapse
*
* \author Siddharth Pritam
*
* @{
*
* This module implements edge collapse of a filtered flag complex, in particular it reduces a filtration of
* Vietoris-Rips complex from its graph to another smaller flag filtration with same persistence.
* Where a filtration is a sequence of simplicial (here Rips) complexes connected with inclusions.
*
* \section edge_collapse_definition Edge collapse definition
*
* An edge \f$e\f$ in a simplicial complex \f$K\f$ is called a <b>dominated edge</b> if the link of \f$e\f$ in
* \f$K\f$, \f$lk_K(e)\f$ is a simplicial cone, that is, there exists a vertex \f$v^{\prime} \notin e\f$ and a
* subcomplex \f$L\f$ in \f$K\f$, such that \f$lk_K(e) = v^{\prime}L\f$. We say that the vertex \f$v^{\prime}\f$ is
* {dominating} \f$e\f$ and \f$e\f$ is {dominated} by \f$v^{\prime}\f$.
* An <b> elementary egde collapse </b> is the removal of a dominated edge \f$e\f$ from \f$K\f$,
* which we denote with \f$K\f$ \f${\searrow\searrow}^1 \f$ \f$K\setminus e\f$.
* The symbol \f$\mathbf{K\setminus e}\f$ (deletion of \f$e\f$ from \f$K\f$) refers to the subcomplex of \f$K\f$ which
* has all simplices of \f$K\f$ except \f$e\f$ and the ones containing \f$e\f$.
* There is an <b>edge collapse</b> from a simplicial complex \f$K\f$ to its subcomplex \f$L\f$,
* if there exists a series of elementary edge collapses from \f$K\f$ to \f$L\f$, denoted as \f$K\f$
* \f${\searrow\searrow}\f$ \f$L\f$.
*
* An edge collapse is a homotopy preserving operation, and it can be further expressed as sequence of the classical
* elementary simple collapse.
* A complex without any dominated edge is called a \f$1\f$- minimal complex and the core \f$K^1\f$ of simplicial
* complex is a minimal complex such that \f$K\f$ \f${\searrow\searrow}\f$ \f$K^1\f$.
* Computation of a core (not unique) involves computation of dominated edges and the dominated edges can be easily
* characterized as follows:
*
* -- For general simplicial complex: An edge \f$e \in K\f$ is dominated by another vertex \f$v^{\prime} \in K\f$,
* <i>if and only if</i> all the maximal simplices of \f$K\f$ that contain \f$e\f$ also contain \f$v^{\prime}\f$
*
* -- For a flag complex: An edge \f$e \in K\f$ is dominated by another vertex \f$v^{\prime} \in K\f$, <i>if and only
* if</i> all the vertices in \f$K\f$ that has an edge with both vertices of \f$e\f$ also has an edge with
* \f$v^{\prime}\f$.
*
* The algorithm to compute the smaller induced filtration is described in Section 5 \cite edgecollapsesocg2020.
* Edge collapse can be successfully employed to reduce any given filtration of flag complexes to a smaller induced
* filtration which preserves the persistent homology of the original filtration and is a flag complex as well.
*
* The general idea is that we consider edges in the filtered graph and sort them according to their filtration value
* giving them a total order.
* Each edge gets a unique index denoted as \f$i\f$ in this order. To reduce the filtration, we move forward with
* increasing filtration value
* in the graph and check if the current edge \f$e_i\f$ is dominated in the current graph \f$G_i := \{e_1, .. e_i\} \f$
* or not.
* If the edge \f$e_i\f$ is dominated we remove it from the filtration and move forward to the next edge \f$e_{i+1}\f$.
* If \f$e_i\f$ is non-dominated then we keep it in the reduced filtration and then go backward in the current graph
* \f$G_i\f$ to look for new non-dominated edges that was dominated before but might become non-dominated at this
* point.
* If an edge \f$e_j, j < i \f$ during the backward search is found to be non-dominated, we include \f$e_j\f$ in to the
* reduced filtration and we set its new filtration value to be \f$i\f$ that is the index of \f$e_i\f$.
* The precise mechanism for this reduction has been described in Section 5 \cite edgecollapsesocg2020.
* Here we implement this mechanism for a filtration of Rips complex.
* After perfoming the reduction the filtration reduces to a flag-filtration with the same persistence as the original
* filtration.
*
* \subsection edgecollapseexample Basic edge collapse
*
* This example calls `Gudhi::collapse::flag_complex_collapse_edges()` from a proximity graph represented as a list of
* `Filtered_edge`.
* Then it collapses edges and displays a new list of `Filtered_edge` (with less edges)
* that will preserve the persistence homology computation.
*
* \include Collapse/edge_collapse_basic_example.cpp
*
* When launching the example:
*
* \code $> ./Edge_collapse_example_basic
* \endcode
*
* the program output is:
*
* \include Collapse/edge_collapse_example_basic.txt
*/
/** @} */ // end defgroup strong_collapse
} // namespace collapse
} // namespace Gudhi
#endif // DOC_EDGE_COLLAPSE_INTRO_EDGE_COLLAPSE_H_
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