# Discrete Morse theory

Discrete Morse theory is a combinatorial adaptation of Morse theory developed by Robin Forman. The theory has various practical applications in diverse fields of applied mathematics and computer science, such as configuration spaces,[1] homology computation,[2][3] denoising,[4] mesh compression,[5] and topological data analysis [6].

## Notation regarding CW complexes

Let ${\displaystyle {\mathcal {X}}}$ be a CW complex. Define the incidence function ${\displaystyle \kappa :{\mathcal {X}}\times {\mathcal {X}}\to \mathbb {Z} }$ in the following way: given two cells ${\displaystyle \sigma }$ and ${\displaystyle \tau }$ in ${\displaystyle {\mathcal {X}}}$, let ${\displaystyle \kappa (\sigma ,~\tau )}$ be the degree of the attaching map from the boundary of ${\displaystyle \sigma }$ to ${\displaystyle \tau }$. The boundary operator ${\displaystyle \partial }$ on ${\displaystyle {\mathcal {X}}}$ is defined by

${\displaystyle \partial (\sigma )=\sum _{\tau \in {\mathcal {X}}}\kappa (\sigma ,\tau )\tau }$

It is a defining property of boundary operators that ${\displaystyle \partial \circ \partial \equiv 0}$. In more axiomatic definitions[7] one can find the requirement that ${\displaystyle \forall \sigma ,\tau ^{\prime }\in {\mathcal {X}}}$

${\displaystyle \sum _{\tau \in {\mathcal {X}}}\kappa (\sigma ,\tau )\kappa (\tau ,\tau ^{\prime })=0}$

which is a corollary of the above definition of the boundary operator and the requirement that ${\displaystyle \partial \circ \partial \equiv 0}$.

## Discrete Morse functions

A real-valued function ${\displaystyle \mu :{\mathcal {X}}\to \mathbb {R} }$ is a discrete Morse function if it satisfies the following two properties:

1. For any cell ${\displaystyle \sigma \in {\mathcal {X}}}$, the number of cells ${\displaystyle \tau \in {\mathcal {X}}}$ in the boundary of ${\displaystyle \sigma }$ which satisfy ${\displaystyle \mu (\sigma )\leq \mu (\tau )}$ is at most one.
2. For any cell ${\displaystyle \sigma \in {\mathcal {X}}}$, the number of cells ${\displaystyle \tau \in {\mathcal {X}}}$ containing ${\displaystyle \sigma }$ in their boundary which satisfy ${\displaystyle \mu (\sigma )\geq \mu (\tau )}$ is at most one.

It can be shown[8] that the cardinalities in the two conditions cannot both be one simultaneously for a fixed cell ${\displaystyle \sigma }$, provided that ${\displaystyle {\mathcal {X}}}$ is a regular CW complex. In this case, each cell ${\displaystyle \sigma \in {\mathcal {X}}}$ can be paired with at most one exceptional cell ${\displaystyle \tau \in {\mathcal {X}}}$: either a boundary cell with larger ${\displaystyle \mu }$ value, or a co-boundary cell with smaller ${\displaystyle \mu }$ value. The cells which have no pairs, i.e., whose function values are strictly higher than their boundary cells and strictly lower than their co-boundary cells are called critical cells. Thus, a discrete Morse function partitions the CW complex into three distinct cell collections: ${\displaystyle {\mathcal {X}}={\mathcal {A}}\sqcup {\mathcal {K}}\sqcup {\mathcal {Q}}}$, where:

1. ${\displaystyle {\mathcal {A}}}$ denotes the critical cells which are unpaired,
2. ${\displaystyle {\mathcal {K}}}$ denotes cells which are paired with boundary cells, and
3. ${\displaystyle {\mathcal {Q}}}$ denotes cells which are paired with co-boundary cells.

By construction, there is a bijection of sets between ${\displaystyle k}$-dimensional cells in ${\displaystyle {\mathcal {K}}}$ and the ${\displaystyle (k-1)}$-dimensional cells in ${\displaystyle {\mathcal {Q}}}$, which can be denoted by ${\displaystyle p^{k}:{\mathcal {K}}^{k}\to {\mathcal {Q}}^{k-1}}$ for each natural number ${\displaystyle k}$. It is an additional technical requirement that for each ${\displaystyle K\in {\mathcal {K}}^{k}}$, the degree of the attaching map from the boundary of ${\displaystyle K}$ to its paired cell ${\displaystyle p^{k}(K)\in {\mathcal {Q}}}$ is a unit in the underlying ring of ${\displaystyle {\mathcal {X}}}$. For instance, over the integers ${\displaystyle \mathbb {Z} }$, the only allowed values are ${\displaystyle \pm 1}$. This technical requirement is guaranteed, for instance, when one assumes that ${\displaystyle {\mathcal {X}}}$ is a regular CW complex over ${\displaystyle \mathbb {Z} }$.

The fundamental result of discrete Morse theory establishes that the CW complex ${\displaystyle {\mathcal {X}}}$ is isomorphic on the level of homology to a new complex ${\displaystyle {\mathcal {A}}}$ consisting of only the critical cells. The paired cells in ${\displaystyle {\mathcal {K}}}$ and ${\displaystyle {\mathcal {Q}}}$ describe gradient paths between adjacent critical cells which can be used to obtain the boundary operator on ${\displaystyle {\mathcal {A}}}$. Some details of this construction are provided in the next section.

## The Morse complex

A gradient path is a sequence of paired cells

${\displaystyle \rho =(Q_{1},K_{1},Q_{2},K_{2},\ldots ,Q_{M},K_{M})}$

satisfying ${\displaystyle Q_{m}=p(K_{m})}$ and ${\displaystyle \kappa (K_{m},~Q_{m+1})\neq 0}$. The index of this gradient path is defined to be the integer

${\displaystyle \nu (\rho )={\frac {\prod _{m=1}^{M-1}-\kappa (K_{m},Q_{m+1})}{\prod _{m=1}^{M}\kappa (K_{m},Q_{m})}}}$.

The division here makes sense because the incidence between paired cells must be ${\displaystyle \pm 1}$. Note that by construction, the values of the discrete Morse function ${\displaystyle \mu }$ must decrease across ${\displaystyle \rho }$. The path ${\displaystyle \rho }$ is said to connect two critical cells ${\displaystyle A,A'\in {\mathcal {A}}}$ if ${\displaystyle \kappa (A,Q_{1})\neq 0\neq \kappa (K_{M},A')}$. This relationship may be expressed as ${\displaystyle A{\stackrel {\rho }{\to }}A'}$. The multiplicity of this connection is defined to be the integer ${\displaystyle m(\rho )=\kappa (A,Q_{1})\cdot \nu (\rho )\cdot \kappa (K_{M},A)}$. Finally, the Morse boundary operator on the critical cells ${\displaystyle {\mathcal {A}}}$ is defined by

${\displaystyle \Delta (A)=\kappa (A,A')+\sum _{A{\stackrel {\rho }{\to }}A'}m(\rho )A'}$

where the sum is taken over all gradient path connections from ${\displaystyle A}$ to ${\displaystyle A'}$.

## Basic Results

Many of the familiar results from continuous Morse theory apply in the discrete setting.

### The Morse Inequalities

Let ${\displaystyle {\mathcal {A}}}$ be a Morse complex associated to the CW complex ${\displaystyle {\mathcal {X}}}$. The number ${\displaystyle m_{q}=|{\mathcal {A}}_{q}|}$ of ${\displaystyle q}$-cells in ${\displaystyle {\mathcal {A}}}$ is called the ${\displaystyle q^{th}}$ Morse number. Let ${\displaystyle \beta _{q}}$ denote the ${\displaystyle q^{th}}$ Betti number of ${\displaystyle {\mathcal {X}}}$. Then, for any ${\displaystyle N>0}$, the following inequalities[9] hold

${\displaystyle m_{N}\geq \beta _{N}}$, and
${\displaystyle m_{N}-m_{N-1}+\ldots \pm m_{0}\geq \beta _{N}-\beta _{N-1}+\ldots \pm \beta _{0}}$

Moreover, the Euler characteristic ${\displaystyle \chi ({\mathcal {X}})}$ of ${\displaystyle {\mathcal {X}}}$ satisfies

${\displaystyle \chi ({\mathcal {X}})=m_{0}-m_{1}+\ldots \pm m_{\dim {\mathcal {X}}}}$

### Discrete Morse Homology and Homotopy Type

Let ${\displaystyle {\mathcal {X}}}$ be a regular CW complex with boundary operator ${\displaystyle \partial }$ and a discrete Morse function ${\displaystyle \mu :{\mathcal {X}}\to \mathbb {R} }$. Let ${\displaystyle {\mathcal {A}}}$ be the associated Morse complex with Morse boundary operator ${\displaystyle \Delta }$. Then, there is an isomorphism[10] of homology groups

${\displaystyle H_{*}({\mathcal {X}},\partial )\simeq H_{*}({\mathcal {A}},\Delta ),}$

and similarly for the homotopy groups.