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Quasi-integrals of motion
By construction, for a Hamiltonian in BGNF is a formal integral of
motion (see section 2.1). We now show how to find
an analogous formal integral for a Hamiltonian in generalized normal form.
Our results are similar to the findings of Meyer and Hall
[19], but the proof differs in some details. We have tried
to make the exposition as transparent as possible by focusing on just
those aspects that are essential
for the reasoning.
We write
as
|
(16) |
and
decompose by means of the Jordan-Chevalley decomposition
[25] into its diagonalizable and nilpotent parts and
:
|
(17) |
Existence
and uniqueness of this decomposition are assured by the Jordan normal form
theorem for matrices.
Define the diagonalizable component
and the
nilpotent component
of
by
|
(18) |
such that
.
We are now in the position to prove the main
Theorem:
For a Hamiltonian
in generalized normal form the
diagonalizable part
of
is a formal integral
of motion.
For the proof we must show that the Poisson bracket of with
vanishes for all . We start with and then proceed to the
case .
By virtue of Jacobi's identity for the Poisson bracket we have
|
(19) |
This expression is zero if
and
commute.
In order to show that the latter is the case we first remark that the
matrices , and are infinitesimally symplectic
[26], i.e. they satisfy (for ).
Direct computation shows that for an infinitesimally symplectic
matrix the Lie operator adjoint to the quadratic polynomial
can be written as
.
Thus we have for the Lie operators and adjoint to
and :
|
(20) |
It is one of the key advantages of this formulation of the theory that we
can characterize all the important operators , and
which operate in a space of the high dimension
by matrices of the considerably smaller dimension
: , and .
We now show that for two commuting matrices , the corresponding
Lie operators
,
(defined as above)
commute as well:
Because
this implies that the right hand side of (27)
is zero, and thus
.
For we proceed in the following way: We show that diagonalizability
and nilpotence of the matrices and carry over to the corresponding
Lie operators and ; these properties then imply that
the null spaces of and coincide and that
, which in turn means
.
Consider a unitary matrix that transforms into the diagonal matrix
.
Inserting twice the identity into the expression for
we get
With
,
and denoting in the new coordinates
by
, we obtain
|
(22) |
Application of this transformed operator to any of the basis monomials
of yields, because is
diagonal, an eigenvalue equation with the eigenfunction
and a certain eigenvalue
--
thus diagonalizability of is shown.
Now consider any .
is a
polynomial in of degree less than or equal to ,
since by nilpotence there is some such that .
This polynomial is related to in the following way:
Iterating this expression and evaluating for we get
|
(23) |
which implies nilpotence of , because (31)
holds for all .
Identity of the null spaces of and is a direct
implication of diagonalizability: Application of the diagonalized operator
(cf. (30)) yields
So the eigenspaces corresponding to the eigenvalue 0 of and
are identical.
Finally, we determine how acts on polynomials in .
Notice that is nilpotent, because its adjoint is.
With
we obtain for any :
because and commute (since the corresponding matrices
and commute; cf. (29)).
is zero for and because
is in generalized normal form. From this it follows for
that
here, for the sake of notational convenience, we have again turned to
the coordinates
as defined above.
Linear independence of the basis monomials
then gives the result
and thus
and we have proven the theorem.
Next: Normalizing a magnetic bottle
Up: Normal forms
Previous: The generalized normal form
  Contents
Martin_Engel
2000-05-25