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Re: Octave criterion for matrix singularity

From: Carlo De Falco
Subject: Re: Octave criterion for matrix singularity
Date: Tue, 26 Feb 2019 13:00:25 +0000

> On 25 Feb 2019, at 19:34, mabalenk <address@hidden> wrote:
> Dear all,
> I'm repeatedly solving a linear system with a Vandermonde matrix. I'm using
> a "backslash" operator:
>  y = -W'/b'
> At each iteration I'm increasing the matrix order by one. At some point
> Octave issues a warning:
>  warning: matrix singular to machine precision, rcond = 1.56508e-17
> If possible, would you please direct me to the Octave source code, where
> this decision is made? I would like to understand the criterion, that
> triggers the warning. I found a post on StackOverflow that says MATLAB would
> issue a similar warning, once rcond < 1e-12. I would like to know, what
> threshold does Octave use and how is it defined. Thank you!
> --
> Best wishes,
> Maxim

The code for solving A\b with A real double precision, full matrix is here :

the condition being checked is

 rcond + 1.0 == 1.0

which means rcond is less or equal than eps(1.0)/2


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