# 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 be any symplectic manifold and

a Hamiltonian on . Let be any regular value of , so that the level set is a smooth manifold. Assume furthermore that is fibered in circles, each of which is an integral curve of the induced Hamiltonian vector field.

Under these assumptions, is a manifold with boundary , and one can form a manifold

by collapsing each circle fiber to a point. In other words, is with the subset removed and the boundary collapsed along each circle fiber. The quotient of the boundary is a submanifold of of codimension two, denoted .

Similarly, one may form from a manifold , which also contains a copy of . The **symplectic cut** is the pair of manifolds and .

Sometimes it is useful to view the two halves of the symplectic cut as being joined along their shared submanifold to produce a singular space

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 be any symplectic manifold. Assume that the circle group acts on in a Hamiltonian way with moment map

This moment map can be viewed as a Hamiltonian function that generates the circle action. The product space , with coordinate on , comes with an induced symplectic form

The group acts on the product in a Hamiltonian way by

with moment map

Let be any real number such that the circle action is free on . Then is a regular value of , and is a manifold.

This manifold contains as a submanifold the set of points with and ; this submanifold is naturally identified with . The complement of the submanifold, which consists of points with , is naturally identified with the product of

and the circle.

The manifold inherits the Hamiltonian circle action, as do its two submanifolds just described. So one may form the symplectic quotient

By construction, it contains as a dense open submanifold; essentially, it compactifies this open manifold with the symplectic quotient

which is a symplectic submanifold of of codimension two.

If is Kähler, then so is the cut space ; however, the embedding of is not an isometry.

One constructs , the other half of the symplectic cut, in a symmetric manner. The normal bundles of in the two halves of the cut are opposite each other (meaning symplectically anti-isomorphic). The symplectic sum of and along recovers .

The existence of a global Hamiltonian circle action on 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 (since the cut is a local operation).

## Blow up as cut

When a complex manifold is blown up along a submanifold , the blow up locus is replaced by an exceptional divisor and the rest of the manifold is left undisturbed. Topologically, this operation may also be viewed as the removal of an -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 be a symplectic manifold with a Hamiltonian -action with moment map . Assume that the moment map is proper and that it achieves its maximum exactly along a symplectic submanifold of . Assume furthermore that the weights of the isotropy representation of on the normal bundle are all .

Then for small the only critical points in are those on . The symplectic cut , which is formed by deleting a symplectic -neighborhood of and collapsing the boundary, is then the symplectic blow up of along .

## References

- Eugene Lerman: Symplectic cuts,
*Mathematical Research Letters*2 (1995), 247–258 - Dusa McDuff and D. Salamon:
*Introduction to Symplectic Topology*(1998) Oxford Mathematical Monographs, ISBN 0-19-850451-9.