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[Octave-bug-tracker] [bug #53576] [octave forge] (symbolic) dsolve error

From: Mike Miller
Subject: [Octave-bug-tracker] [bug #53576] [octave forge] (symbolic) dsolve errors on sqrt coefficient
Date: Fri, 6 Apr 2018 13:41:36 -0400 (EDT)
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:59.0) Gecko/20100101 Firefox/59.0

Update of bug #53576 (project octave):

                 Summary: Symbolic pkg dsolve now not woeking => [octave
forge] (symbolic) dsolve errors on sqrt coefficient


Follow-up 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):

de = (sym)

                    2           3          
  d               dx           d           
  ──(y(x)) + ────────────── + ───(y(x))
= 0
  dx              985           3          

In the current symbolic 2.6.0, it is

de = (sym)

                  2           3          
  d              d           d           
  ──(y(x)) + √2⋅───(y(x)) + ───(y(x)) = 0
  dx              2           3          
                dx          dx           

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.


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