[Numpy-discussion] On the difference of two positive definite matrices

denis denis-bz-gg at t-online.de
Mon Dec 17 06:52:57 EST 2012

Charles R Harris <charlesr.harris <at> gmail.com> writes:
> On Sun, Dec 16, 2012 at 6:20 PM, Virgil Stokes <vs <at> it.uu.se> wrote:
> Suppose I have two positive definite matrices, A and B. Is it possible
> to use U*D*U^T  factorizations of these matrices to obtain a numerically
> stable result for their difference, A - B ?
> Not sure what you are asking, but there is a coordinate system in which they
> are both diagonal. Nevertheless, the difference may not be positive 
> definite.Chuck 

shows how to do that, but
"note that this is no longer an orthogonal diagonalization";
orthogonal can do it iff A and B commute.
  -- denis

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