In differential geometry, a branch of mathematics, the Moser's trick (or Moser's argument) is a method to relate two differential forms
and
on a smooth manifold by a diffeomorphism
such that
, provided that one can find a family of vector fields satisfying a certain ODE.
More generally, the argument holds for a family
and produce an entire isotopy
such that
.
It was originally given by Jürgen Moser in 1965 to check when two volume forms are equivalent,[1] but its main applications are in symplectic geometry. It is the standard argument for the modern proof of Darboux's theorem, as well as for the proof of Darboux-Weinstein theorem[2] and other normal form results.[2][3][4]
Let
be a family of differential forms on a compact manifold
. If the ODE
admits a solution
, then there exists a family
of diffeomorphisms of
such that
and
.
In particular, there is a diffeomorphism
such that
.
The trick consists in viewing
as the flows of a time-dependent vector field, i.e. of a smooth family
of vector fields on
. Using the definition of flow, i.e.
for every
, one obtains from the chain rule that
By hypothesis, one can always find
such that
, hence their flows
satisfies
. In particular, as
is compact, this flows exists at
.
Let
be two volume forms on a compact
-dimensional manifold
. Then there exists a diffeomorphism
of
such that
if and only if
.[1]
One implication holds by the invariance of the integral by diffeomorphisms:
.
For the converse, we apply Moser's trick to the family of volume forms
. Since
, the de Rham cohomology class
vanishes, as a consequence of Poincaré duality and the de Rham theorem. Then
for some
, hence
. By Moser's trick, it is enough to solve the following ODE, where we used the Cartan's magic formula, and the fact that
is a top-degree form:
However, since
is a volume form, i.e.
, given
one can always find
such that
.
Application to symplectic structures
[edit]
In the context of symplectic geometry, the Moser's trick is often presented in the following form.[3][4]
Let
be a family of symplectic forms on
such that
, for
. Then there exists a family
of diffeomorphisms of
such that
and
.
In order to apply Moser's trick, we need to solve the following ODE
where we used the hypothesis, the Cartan's magic formula, and the fact that
is closed. However, since
is non-degenerate, i.e.
, given
one can always find
such that
.
Given two symplectic structures
and
on
such that
for some point
, there are two neighbourhoods
and
of
and a diffeomorphism
such that
and
.[3][4]
This follows by noticing that, by Poincaré lemma, the difference
is locally
for some
; then, shrinking further the neighbourhoods, the result above applied to the family
of symplectic structures yields the diffeomorphism
.
Darboux theorem for symplectic structures
[edit]
The Darboux's theorem for symplectic structures states that any point
in a given symplectic manifold
admits a local coordinate chart
such that
While the original proof by Darboux required a more general statement for 1-forms,[5] Moser's trick provides a straightforward proof. Indeed, choosing any symplectic basis of the symplectic vector space
, one can always find local coordinates
such that
. Then it is enough to apply the corollary of Moser's trick discussed above to
and
, and consider the new coordinates
.[3][4]
Application: Moser stability theorem
[edit]
Moser himself provided an application of his argument for the stability of symplectic structures,[1] which is known now as Moser stability theorem.[3][4]
Let
a family of symplectic form on
which are cohomologous, i.e. the deRham cohomology class
does not depend on
. Then there exists a family
of diffeomorphisms of
such that
and
.
It is enough to check that
; then the proof follows from the previous application of Moser's trick to symplectic structures. By the cohomologous hypothesis,
is an exact form, so that also its derivative
is exact for every
. The actual proof that this can be done in a smooth way, i.e. that
for a smooth family of functions
, requires some algebraic topology. One option is to prove it by induction, using Mayer-Vietoris sequences;[3] another is to choose a Riemannian metric and employ Hodge theory.[1]