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Topic: Eigensystem ignoring assumptions
Replies: 3   Last Post: Feb 1, 2014 12:51 AM

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Matthias Bode

Posts: 84
Registered: 3/16/05
Re: Eigensystem ignoring assumptions
Posted: Feb 1, 2014 12:49 AM
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"references to the documentation":

"$Assumptions is the default setting for the Assumptions option used in such functions as Simplify, Refine, and Integrate."

"Assumptions is an option for functions such as Simplify, Refine, and Integrate that specifies default assumptions to be made about symbolic quantities."

"Assuming[assum,expr] evaluates expr with assum appended to $Assumptions, so that assum is included in the default assumptions used by functions such as Refine, Simplify, and Integrate."

Apparently Eigensystem[] has, as yet, not come within in the ambit of "functions such as".

Best regards,

S 17.36398, W 66.21816,2'590 m. AMSL.
> From:
> Subject: Eigensystem ignoring assumptions
> To:
> Date: Thu, 30 Jan 2014 23:04:40 -0500
> Hi,
> I am trying to understand, why Eigensystem is ignoring the assumptions and
> still dividing by something that is explicitly assumed to be zero. This is
> a more extreme counterpart of the b>=0 case. Any comments or references to
> the documentation are appreciated.
> In[320]:= Clear["Global`*"]
> In[321]:= $Assumptions = b == 0;
> In[322]:= m1 = ({
> {a, b},
> {b, -a}
> });
> In[323]:= Eigensystem[m1]
> Out[323]= {{-Sqrt[a^2 + b^2], Sqrt[
> a^2 + b^2]}, {{-((-a + Sqrt[a^2 + b^2])/b),
> 1}, {-((-a - Sqrt[a^2 + b^2])/b), 1}}}

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