# Symplectic cut

In mathematics, specifically in symplectic geometry, the symplectic cut is a geometric modification on symplectic manifolds. Its effect is to decompose a given manifold into two pieces. There is an inverse operation, the symplectic sum, that glues two manifolds together into one. The symplectic cut can also be viewed as a generalization of symplectic blow up. The cut was introduced in 1995 by Eugene Lerman, who used it to study the symplectic quotient and other operations on manifolds.

## Topological description

Let $(X,\omega )$ be any symplectic manifold and
    $\mu :X\to \mathbb {R}$
a Hamiltonian on $X$. Let $\epsilon$ be any regular value of $\mu$, so that the level set $\mu ^{-1}(\epsilon )$ is a smooth manifold. Assume furthermore that $\mu ^{-1}(\epsilon )$ is fibered in circles, each of which is an integral curve of the induced Hamiltonian vector field.
Under these assumptions, $\mu ^{-1}([\epsilon ,\infty ))$ is a manifold with boundary $\mu ^{-1}(\epsilon )$, and one can form a manifold
    ${\overline {X}}_{\mu \geq \epsilon }$
by collapsing each circle fiber to a point. In other words, ${\overline {X}}_{\mu \geq \epsilon }$ is $X$ with the subset $\mu ^{-1}((-\infty ,\epsilon ))$ removed and the boundary collapsed along each circle fiber. The quotient of the boundary is a submanifold of ${\overline {X}}_{\mu \geq \epsilon }$ of codimension two, denoted $V$.
Similarly, one may form from $\mu ^{-1}((-\infty ,\epsilon ])$ a manifold ${\overline {X}}_{\mu \leq \epsilon }$, which also contains a copy of $V$. The symplectic cut is the pair of manifolds ${\overline {X}}_{\mu \leq \epsilon }$ and ${\overline {X}}_{\mu \geq \epsilon }$.
Sometimes it is useful to view the two halves of the symplectic cut as being joined along their shared submanifold $V$ to produce a singular space
    ${\overline {X}}_{\mu \leq \epsilon }\cup _{V}{\overline {X}}_{\mu \geq \epsilon }.$
For example, this singular space is the central fiber in the symplectic sum regarded as a deformation.

## Symplectic description

The preceding description is rather crude; more care is required to keep track of the symplectic structure on the symplectic cut. For this, let $(X,\omega )$ be any symplectic manifold. Assume that the circle group $U(1)$ acts on $X$ in a Hamiltonian way with moment map
    $\mu :X\to \mathbb {R} .$
This moment map can be viewed as a Hamiltonian function that generates the circle action. The product space $X\times \mathbb {C}$, with coordinate $z$ on $\mathbb {C}$, comes with an induced symplectic form
    $\omega \oplus (-idz\wedge d{\bar {z}}).$
The group $U(1)$ acts on the product in a Hamiltonian way by
    $e^{i\theta }\cdot (x,z)=(e^{i\theta }\cdot x,e^{-i\theta }z)$
with moment map
    $\nu (x,z)=\mu (x)-|z|^{2}.$
Let $\epsilon$ be any real number such that the circle action is free on $\mu ^{-1}(\epsilon )$. Then $\epsilon$ is a regular value of $\nu$, and $\nu ^{-1}(\epsilon )$ is a manifold.
This manifold $\nu ^{-1}(\epsilon )$ contains as a submanifold the set of points $(x,z)$ with $\mu (x)=\epsilon$ and $|z|^{2}=0$; this submanifold is naturally identified with $\mu ^{-1}(\epsilon )$. The complement of the submanifold, which consists of points $(x,z)$ with $\mu (x)>\epsilon$, is naturally identified with the product of
    $X_{>\epsilon }:=\mu ^{-1}((\epsilon ,\infty ))$
and the circle.
The manifold $\nu ^{-1}(\epsilon )$ inherits the Hamiltonian circle action, as do its two submanifolds just described. So one may form the symplectic quotient
    ${\overline {X}}_{\mu \geq \epsilon }:=\nu ^{-1}(\epsilon )/U(1).$
By construction, it contains $X_{\mu >\epsilon }$ as a dense open submanifold; essentially, it compactifies this open manifold with the symplectic quotient
    $V:=\mu ^{-1}(\epsilon )/U(1),$
which is a symplectic submanifold of ${\overline {X}}_{\mu \geq \epsilon }$ of codimension two.
If $X$ is Kähler, then so is the cut space ${\overline {X}}_{\mu \geq \epsilon }$; however, the embedding of $X_{\mu >\epsilon }$ is not an isometry.
One constructs ${\overline {X}}_{\mu \leq \epsilon }$, the other half of the symplectic cut, in a symmetric manner. The normal bundles of $V$ in the two halves of the cut are opposite each other (meaning symplectically anti-isomorphic). The symplectic sum of ${\overline {X}}_{\mu \geq \epsilon }$ and ${\overline {X}}_{\mu \leq \epsilon }$ along $V$ recovers $X$.
The existence of a global Hamiltonian circle action on $X$ appears to be a restrictive assumption. However, it is not actually necessary; the cut can be performed under more general hypotheses, such as a local Hamiltonian circle action near $\mu ^{-1}(\epsilon )$ (since the cut is a local operation).

## Blow up as cut

When a complex manifold $X$ is blown up along a submanifold $Z$, the blow up locus $Z$ is replaced by an exceptional divisor $E$ and the rest of the manifold is left undisturbed. Topologically, this operation may also be viewed as the removal of an $\epsilon$-neighborhood of the blow up locus, followed by the collapse of the boundary by the Hopf map.
Blowing up a symplectic manifold is more subtle, since the symplectic form must be adjusted in a neighborhood of the blow up locus in order to continue smoothly across the exceptional divisor in the blow up. The symplectic cut is an elegant means of making the neighborhood-deletion/boundary-collapse process symplectically rigorous.
As before, let $(X,\omega )$ be a symplectic manifold with a Hamiltonian $U(1)$-action with moment map $\mu$. Assume that the moment map is proper and that it achieves its maximum $m$ exactly along a symplectic submanifold $Z$ of $X$. Assume furthermore that the weights of the isotropy representation of $U(1)$ on the normal bundle $N_{X}Z$ are all $1$.
Then for small $\epsilon$ the only critical points in $X_{\mu >m-\epsilon }$ are those on $Z$. The symplectic cut ${\overline {X}}_{\mu \leq m-\epsilon }$, which is formed by deleting a symplectic $\epsilon$-neighborhood of $Z$ and collapsing the boundary, is then the symplectic blow up of $X$ along $Z$.