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# QZ factorization in Mathematica
I am trying to track down a Mathematica version of the QZ algorithm.
This algorithm was first published by Moler and Stewart (1973), "An
algorithm for generalised matrix eigenvalue problems" SIAM Journal on
Numerical Analysis 10. pp 241-256, and apparently it also appears in
Coleman and van Loan (1988) Handbook for matrix computation.
According to the paper I am reading, the factorization works like this
\!\(\[CapitalLambda]\ = \ Q\ \(\[CapitalGamma]\_0\) Z\)
\!\(\[CapitalOmega]\ = \ Q\ \(\[CapitalGamma]\_1\) Z\)
( Lambda = Q. Gamma0. Z and Omega = Q. Gamma1. Z )
where Gamma1 and Gamma0 are known square numerical matrices.
CapitalLambda and CapitalOmega are upper triangular. Transpose[Q].Q
== IdentityMatrix[n] and also Transpose[Z].Z == IdentityMatrix[n]
As far as I can work out from the numerical example in the paper that
I'm reading this from, Z is the SchurDecomposition of
Inverse[Gamma0].Gamma1
(the paper is H.M. Amman and D.A. Kendrick (1998) "Computing the
steady state of linear quadratic optimization models with rational
expectations", Economics Letters 58 pp 185-191, if anyone's interested)
But I can't work out what Q is and therefore I can't work out what
CapitalLambda and CapitalOmega are, which renders the rest of the paper
pretty useless. The only other pieces of information I have about the
problem is that the ratios of the elements on the lead diagonals of
CapitalOmega/CapitalLambda equal Eigenvalues[Inverse[Gamma0].Gamma1].
I tried using the fact that various elements of CapitalLambda and
CapitalOmega are zero (because they are upper triangular) to Solve for
the elements of Q, but I couldn't get it to work and it must be a bit
ugly to write a general version of such a procedure.
Has anyone implemented this factorization in Mathematica? I know it
exists in another program, but I don't have it and would rather not give
money to a company that has broadcast its intention to abandon my
platform of choice.
A complex-matrix version of the algorithm was published in
"Transactions in Mathematical Software" as Algorithm 535: it is
available on the web at
http://www.netlib.org/toms/535
and probably elsewhere. Unfortunately that version is in Fortran, and
although I can vaguely work out what's going on, I don't trust my
understanding of either the mathematics or the Fortran language to
translate it adequately.
Please email me directly as well as posting any reply.
Thanks in advance,
Luci Ellis
elisha@remove-to-reply.dot.net.au
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