bugGNU Octave - Bugs: bug #53576, [octave forge] (symbolic) dsolve...

 
 

bug #53576: [octave forge] (symbolic) dsolve errors on sqrt coefficient

Submitted by:  Doug Stewart <dastew>
Submitted on:  Fri 06 Apr 2018 04:24:39 PM UTC  
 
Category: Octave Forge PackageSeverity: 3 - Normal
Priority: 1 - LaterItem Group: Unexpected Error
Status: PostponedAssigned to: None
Originator Name: Doug StewartOpen/Closed: Open
Release: 4.2.2Operating System: GNU/Linux

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Mon 09 Apr 2018 09:55:39 PM UTC, comment #6:

I've filed https://github.com/cbm755/octsympy/issues/883 with a Python example showing the same error. One of us will chase this up to SymPy.

Mike Miller <mtmiller>
Project Administrator
Fri 06 Apr 2018 06:51:27 PM UTC, comment #5:

I would like to see it fixed in sympy.
I think that sqrt(2) is used in so many places that any symbolic code should handle it.

Doug Stewart <dastew>
Fri 06 Apr 2018 06:07:01 PM UTC, comment #4:

Should I close this as not-a-bug or do you think we should pursue getting this error fixed in SymPy?

Mike Miller <mtmiller>
Project Administrator
Fri 06 Apr 2018 06:03:27 PM UTC, comment #3:

Mike you are correct. thanks for looking at this.
I will change the wiki to
sqrt2=sym(1.41421);
de =diff(y, 3 ) +sqrt2*diff(y,2) + diff(y) == 0;

Doug Stewart <dastew>
Fri 06 Apr 2018 05:47:38 PM UTC, comment #2:

In version 2.5.0, if the equation was written as

then it would have failed with the same error. Thus I am changing this from regression.

Mike Miller <mtmiller>
Project Administrator
Fri 06 Apr 2018 05:41:36 PM UTC, comment #1:

I think the relevant piece that has changed here is that the symbolic package now tries to work around any floating point values it sees by applying various heuristics to convert it into a symbolic expression.

In version 2.5.0, the DE was converted into this symbolic expression, with a rational approximation to sqrt(2):

In the current symbolic 2.6.0, it is

And the sqrt(2), which wasn't there before, seems to be where SymPy's dsolve is erroring.

If you convert the coefficient to some other number, it will work again. It would be best to show an example with all integer or rational coefficients.

Mike Miller <mtmiller>
Project Administrator
Fri 06 Apr 2018 04:24:39 PM UTC, original submission:

running this code from octave wiki symbolic demos now fails.

with this error:

I am using ubuntu 16.04

Doug Stewart <dastew>

 

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    Follow 4 latest changes.

    Date Changed By Updated Field Previous Value => Replaced By
    Mon 09 Apr 2018 09:55:39 PM UTCmtmillerPriority5 - Normal=>1 - Later
      StatusNone=>Postponed
    Fri 06 Apr 2018 05:47:38 PM UTCmtmillerItem GroupRegression=>Unexpected Error
    Fri 06 Apr 2018 05:41:36 PM UTCmtmillerSummarySymbolic pkg dsolve now not woeking=>[octave forge] (symbolic) dsolve errors on sqrt coefficient

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