#11934: Symbolic simplification error
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Reporter: mjo | Owner: burcin
Type: defect | Status: needs_info
Priority: major | Milestone: sage-5.11
Component: symbolics | Resolution:
Keywords: | Work issues:
Report Upstream: N/A | Reviewers:
Authors: Michael Orlitzky | Merged in:
Dependencies: #12322 | Stopgaps:
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Comment (by mjo):
Replying to [comment:6 kcrisman]:
> Needs work/info, but not because of your patch or #12737, but rather
because this doesn't really treat the underlying issue. This could just
be some floating point thing that is inherently impossible to avoid once
one allows complex numbers (and since your original example is complex
sometimes, the answers are going to be complex, unfortunately).
There aren't any floating point issues if you don't call `n()` on
anything. The issue is still there with `simplify_radical()`, but that's
because `simplify_radical()` is broken by design: it chooses a branch
arbitrarily for the square root. This is what `radcan()` in Maxima is
documented to do, but if you re-brand it as a simplification, it's a bug.
Plain `simplify()` works:
{{{
sage: f = sqrt(-8*(4*sqrt(2) - 7)*x^4 + 16*(3*sqrt(2) - 5)*x^3)
sage: f.simplify()
sqrt(-8*(4*sqrt(2) - 7)*x^4 + 16*(3*sqrt(2) - 5)*x^3)
}}}
As does `simplify_trig()`:
{{{
sage: f.simplify_trig()
sqrt(-8*(4*sqrt(2) - 7)*x^4 + 16*(3*sqrt(2) - 5)*x^3)
}}}
And `simplify_rational()`:
{{{
sage: f.simplify_rational()
sqrt(-8*(4*sqrt(2) - 7)*x^4 + 16*(3*sqrt(2) - 5)*x^3)
}}}
And `simplify_log()`:
{{{
sage: f.simplify_log()
sqrt(-8*(4*sqrt(2) - 7)*x^4 + 16*(3*sqrt(2) - 5)*x^3)
}}}
It's only `simplify_radical()` that messes everything up (note I haven't
called `n()` anywhere, so there are no numerical issues):
{{{
sage: f.simplify_radical()
2*I*sqrt((4*sqrt(2) - 7)*x - 6*sqrt(2) + 10)*sqrt(2)*x^(3/2)
}}}
I don't want a random branch of the square root when I ask for a
simplification. I want a simplification, but only if possible! Without
more information, you can't simplify that expression. The rest of the
simplify functions don't do anything, but that's the correct thing to do
here.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/11934#comment:7>
Sage <http://www.sagemath.org>
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