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In mathematics, the Redheffer star product is a binary operation on linear operators that arises in connection to solving coupled systems of linear equations. It was introduced by Raymond Redheffer in 1959,[1] and has subsequently been widely adopted in computational methods for scattering matrices. Given two scattering matrices from different linear scatterers, the Redheffer star product yields the combined scattering matrix produced when some or all of the output channels of one scatterer are connected to inputs of another scatterer.
Definition
Suppose are the block matrices
and
,
whose blocks have the same shape when
.
The Redheffer star product is then defined by:
[1]
,
assuming that are invertible,
where is an identity matrix conformable
to or , respectively.
This can be rewritten several ways making use of the so-called
push-through identity.
Redheffer's definition extends beyond matrices to
linear operators on a Hilbert space.
[2]
.
By definition, are linear endomorphisms of ,
making linear endomorphisms of ,
where is the direct sum.
However, the star product still makes sense as long as the transformations are compatible,
which is possible when
and
so that .
Properties
Existence
exists if and only if
exists.
[3]
Thus when either exists, so does the Redheffer star product.
The star product is associative, provided all of the relevant matrices are defined.
[3]
Thus .
Adjoint
Provided either side exists, the adjoint of a Redheffer
star product is .
[2]
Inverse
If is the left matrix inverse of such that
, has a right inverse, and
exists, then .
[2]
Similarly, if is the left matrix inverse of such
that , has a right inverse, and
exists, then .
The coupled system of equations, with arrows labeling the inputs and outputs to each matrix
The star product arises from solving multiple linear systems of equations that share
variables in common.
Often, each linear system models the behavior of one subsystem in a physical process
and by connecting the multiple subsystems into a whole, one can eliminate variables
shared across subsystems in order to obtain the overall linear system.
For instance, let be elements of a Hilbert space such that
[4]
and
The "plumbing" of one of Redheffer's systems of equations.
giving the following equations in variables:
.
By substituting the first equation into the last we find:
.
By substituting the last equation into the first we find:
.
Eliminating by substituting the two preceding equations
into those for results in the Redheffer star product
being the matrix such that:
[1]
The star product eliminates the shared variables in this coupled system of equations.
.
Connection to scattering matrices
The "plumbing" of the scattering matrix has a different convention than Redheffer that amounts to swapping and relabeling several quantities. The advantage is that now the S-matrix's subscripts label the input and output ports as well as the block indices.
Many scattering processes take on a form that motivates a different
convention for the block structure of the linear system of a scattering matrix.
Typically a physical device that performs a linear transformation on inputs, such as
linear dielectric media on electromagnetic waves or in quantum mechanical scattering,
can be encapsulated as a system which interacts with the environment through various
ports, each of which accepts inputs and returns outputs. It is conventional to use a different notation for the Hilbert space, , whose subscript
labels a port on the device.
Additionally, any element, , has an additional superscript labeling the direction of travel (where + indicates moving from port i to i+1 and - indicates the reverse).
The equivalent notation for a Redheffer transformation,
,
used in the previous section is
.
The action of the S-matrix,
,
is defined with an additional flip compared to Redheffer's definition:[5]
,
so
.
Note that for in order for the off-diagonal identity matrices to be defined,
we require be the same underlying Hilbert space.
(The subscript does not imply any difference, but is just a label for bookkeeping.)
The star product, ,
for two S-matrices, , is given by
[5]
The "plumbing" of a coupled pair of scattering matrices in a star product.
,
where
and ,
so .
Properties
These are analogues of the properties of for
Most of them follow from the correspondence
.
, the exchange operator, is also the S-matrix star identity defined below.
For the rest of this section, are S-matrices.
Existence
exists when either
or
exist.
Identity
The S-matrix star identity, , is
.
This means
Associativity
Associativity of follows from associativity of and of matrix multiplication.
Adjoint
From the correspondence between and ,
and the adjoint of , we have that
Inverse
The matrix that is the S-matrix star product inverse of
in the sense that
is where is the ordinary matrix inverse
and is as defined above.
Connection to transfer matrices
Transfer matrices have a different "plumbing" than scattering matrices. They connect one port to another instead of the inputs at all ports to the outputs at all ports.
Observe that a scattering matrix can be rewritten as a
transfer matrix, , with action
,
where
[6]
.
Here the subscripts relate the different directions of propagation at each port.
As a result, the star product of scattering matrices
,
is analogous to the following matrix multiplication of transfer matrices
[7]
,
where
and ,
so .
Generalizations
Redheffer generalized the star product in several ways:
Arbitrary bijections
If there is a bijection given by
then an associative star product can be defined by:
[7]
.
The particular star product defined by Redheffer above is obtained from:
where .
3x3 star product
A star product can also be defined for 3x3 matrices.
[8]
Applications to scattering matrices
In physics, the Redheffer star product appears when constructing a total
scattering matrix from two or more subsystems.
If system has a scattering matrix and system
has scattering matrix , then the combined system
has scattering matrix .
[5]
Transmission line theory
Many physical processes, including radiative transfer, neutron diffusion, circuit theory, and others are described by scattering processes whose formulation depends on the dimension of the process and the representation of the operators.[6] For probabilistic problems, the scattering equation may appear in a Kolmogorov-type equation.
Electromagnetism
The Redheffer star product can be used to solve for the propagation of electromagnetic fields in stratified, multilayered media.[9] Each layer in the structure has its own scattering matrix and the total structure's scattering matrix can be described as the star product between all of the layers.[10] A free software program that simulates electromagnetism in layered media is the
Stanford Stratified Structure Solver.
Semiconductor interfaces
Kinetic models of consecutive semiconductor interfaces can use a scattering matrix formulation to model the motion of electrons between the semiconductors.
[11]
Factorization on graphs
In the analysis of Schrödinger operators on graphs, the scattering matrix of a graph can be obtained as a generalized star product of the scattering matrices corresponding to its subgraphs.[12]