GenericSchur

The Schur decomposition is the workhorse for eigensystem analysis of dense matrices. The diagonal eigen-decomposition of normal (especially Hermitian) matrices is an important special case, but for non-normal matrices the Schur form is often more useful.

The purpose of this package is to extend the schur! and related functions of the standard library to number types not handled by LAPACK, such as Complex{BigFloat}, Complex{Float128} (from Quadmath.jl), etc. For these, the schur!, eigvals!, and eigen! functions in the LinearAlgebra standard library are overloaded here, and may be accessed through the usual schur, eigvals, and eigen wrappers:

A = your_matrix_generator()
S = schur(A)

The result S is a LinearAlgebra.Schur object, with the properties T, Z=vectors, and values.

The unexported gschur and gschur! functions are available for types normally handled by the LAPACK wrappers in LinearAlgebra.

For square matrices of complex element type, this package provides a full Schur decomposition:

A::StridedMatrix{C} where {C <: Complex} == Z * T * adjoint(Z)

where T is upper-triangular and Z is unitary, both with the same element type as A. (See below for real matrices.)

The algorithm is essentially the unblocked, serial, single-shift Francis (QR) scheme used in the complex LAPACK routines. Balancing is also available.

Right and left eigenvectors are available from complex Schur factorizations, using

S = schur(A)
VR = eigvecs(S)
VL = eigvecs(S,left=true)

As of v0.4, eigenvectors as returned from our eigen and eigvecs methods for the standard problem have unit Euclidean norm. This accords with the current (undocumented) behavior of LinearAlgebra methods. (Previously a convention based on low-level LAPACK routines was used here.)

A quasi-triangular "real Schur" decomposition of real matrices is also provided:

A::StridedMatrix{R} where {R <: Real} == Z * T * transpose(Z)

where T is quasi-upper-triangular and Z is orthogonal, both with the same element type as A. This is what you get by invoking the above-mentioned functions with matrix arguments whose element type T <: Real. The result is in standard form, so pair-blocks (and therefore rank-2 invariant subspaces) should be fully resolved.

Eigenvectors are not currently available for the "real Schur" forms. But don't despair; one can convert a standard quasi-triangular real Schur into a complex Schur with the triangularize function provided here.

The accuracy of eigenvalues and eigenvectors may be improved for some matrices by use of a similarity transform which reduces the matrix norm. This is done by default in the eigen! method, and may also be handled explicitly via the balance! function provided here:

Ab, B = balance!(copy(A))
S = schur(Ab)
v = eigvecs(S)
lmul!(B, v) # to get the eigenvectors of A

More details are in the function docstring. Although the balancing function also does permutations to isolate trivial subspaces, the Schur routines do not yet exploit this opportunity for reduced workload.

Methods for reordering a Schur decomposition (ordschur) and computing condition numbers (eigvalscond) and subspace separation (subspacesep) are provided. Tests to date suggest that behavior is analogous to the LAPACK routines on which the implementation is based.

Methods for the generalized eigenvalue problem (matrix pencils) with Complex element types are available as of release 0.3.0; in particular, extension of schur(A,B) from LinearAlgebra. The algorithms are translated from LAPACK, but this implementation has had limited testing. (Note that it is easy to check the decomposition of a particular case ex post facto.)

Corresponding functions for reordering and condition estimation are included. Tests to date suggest that behavior is analogous to LAPACK.

Right eigenvectors of generalized problems are available with V = eigvecs(S::GeneralizedSchur{<:Complex}). Column j of V satisfies S.beta[j] * A * v ≈ S.alpha[j] * B * v. These currently have a peculiar norm intended to be compatible with LAPACK conventions.

This package incorporates or elaborates several methods from Andreas Noack's GenericLinearAlgebra.jl package, and includes translations from LAPACK code.