#12807: Taking the real part of a sum of exponentials with imaginary exponents
gives wrong result
-------------------------------------------------+-------------------------
Reporter: inaki | Owner: burcin
Type: defect | Status: new
Priority: major | Milestone: sage-6.7
Component: symbolics | Resolution:
Keywords: | Merged in:
Authors: | Reviewers:
Report Upstream: Fixed upstream, in a later | Work issues:
stable release. | Commit:
Branch: | Stopgaps:
Dependencies: pynac-0.3.7 |
-------------------------------------------------+-------------------------
Changes (by rws):
* upstream: N/A => Fixed upstream, in a later stable release.
* dependencies: => pynac-0.3.7
* milestone: sage-6.4 => sage-6.7
Comment:
Actually and surprisingly what's happening is that the simplified values
of the `exp` expressions (`1/2` and `-1/2`) and their coefficients are all
multiplied together on final evaluation:
{{{
sage: (4*exp(i*pi/3)-4*exp(i*2*pi/3)).real_part()
4
sage: (6*exp(i*pi/3)-6*exp(i*2*pi/3)).real_part()
9
}}}
This is evident in the GiNaC source, this
line(https://github.com/pynac/pynac/blob/master/ginac/add.cpp#L432):
{{{
oc = oc.mul(ex_to<numeric>(j->rest)).mul(ex_to<numeric>(j->coeff));
}}}
is so rarely executed that e.g. only three doctests in `symbolic/` touch
it but don't trigger the bug because the expressions are too short.
The fix will be in Pynac-0.3.7.
--
Ticket URL: <http://trac.sagemath.org/ticket/12807#comment:7>
Sage <http://www.sagemath.org>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica,
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