Schur class explained
defined on the open unit disk
and satisfying
that solve the Schur problem: Given complex numbers
, find a function
which is analytic and bounded by on the unit disk. The method of solving this problem as well as similar problems (e.g. solving Toeplitz systems and
Nevanlinna-Pick interpolation) is known as the
Schur algorithm (also called
Coefficient stripping or
Layer stripping). One of the algorithm's most important properties is that it generates
orthogonal polynomials which can be used as orthonormal basis functions to expand any th-order polynomial.
[1] It is closely related to the
Levinson algorithm though Schur algorithm is numerically more stable and better suited to parallel processing.
[2] Schur function
Consider the Carathéodory function of a unique probability measure
on the unit circle
given by
where
implies
. Then the association
sets up a one-to-one correspondence between Carathéodory functions and Schur functions
given by the inverse formula:
Schur algorithm
Schur's algorithm is an iterative construction based on Möbius transformations that maps one Schur function to another.[3] The algorithm defines an infinite sequence of Schur functions
f\equivf0,f1,...c,fn,...c
and
Schur parameters \gamma0,\gamma1,...c,\gamman,...c
(also called
Verblunsky coefficient or
reflection coefficient) via the recursion:
fj+1=
| fj(z)-\gammaj |
1-\overline{\gammaj |
fj(z)}, fj(0)\equiv\gammaj\inD,
which stops if
fj(z)\equivei\theta=\gammaj\inT
. One can invert the transformation as
f(z)\equivf0(z)=
| \gamma0+zf1(z) |
1+\overline{\gamma0 |
zf1(z)}
or, equivalently, as
continued fraction expansion of the Schur function
}}by repeatedly using the fact that
}.
See also
Notes and References
- Book: Chung . Jin-Gyun . Parhi . Keshab K. . The Kluwer International Series in Engineering and Computer Science . Pipelined Lattice and Wave Digital Recursive Filters . Springer US . Boston, MA . 1996 . 978-1-4612-8560-1 . 0893-3405 . 10.1007/978-1-4613-1307-6 . 79.
- Book: Hayes, Monson H.. Statistical digital signal processing and modeling . 242. 1996 . John Wiley & Son . 978-0-471-59431-4. 34243409.
- Book: Conway, John B. . Functions of One Complex Variable I (Graduate Texts in Mathematics 11) . Graduate Texts in Mathematics . Springer-Verlag . 1978 . 978-0-387-90328-6. 127.