Opened 9 years ago
Closed 6 years ago
#12807 closed defect (fixed)
Taking the real part of a sum of exponentials with imaginary exponents gives wrong result
Reported by: | inaki | Owned by: | burcin |
---|---|---|---|
Priority: | major | Milestone: | sage-6.7 |
Component: | symbolics | Keywords: | |
Cc: | Merged in: | ||
Authors: | Ralf Stephan | Reviewers: | Travis Scrimshaw |
Report Upstream: | Fixed upstream, in a later stable release. | Work issues: | |
Branch: | 410bab7 (Commits, GitHub, GitLab) | Commit: | 410bab7e7016f06220e1a34335abbaa8f5e3e876 |
Dependencies: | #18362 | Stopgaps: |
Description
In sage 4.8 the following computation gives 1/2, while the right result is -3/2:
sage: a = exp(i*2*pi/3) sage: b = exp(i*pi/3) sage: c = 2*a-b sage: c.real() 1/2
Doing the computation numerically gives the right result:
sage: N(c).real() -1.50000000000000
and asking maxima directly also works:
sage: (c.maxima_methods().rectform()).real() -3/2
This is sage 4.8 running on Fedora 16, linux 3.1.0-7.fc16.x86_64 SMP.
Change History (11)
comment:1 Changed 9 years ago by
comment:2 Changed 8 years ago by
- Milestone changed from sage-5.11 to sage-5.12
comment:3 Changed 7 years ago by
- Milestone changed from sage-6.1 to sage-6.2
comment:4 Changed 7 years ago by
- Milestone changed from sage-6.2 to sage-6.3
comment:5 Changed 7 years ago by
- Milestone changed from sage-6.3 to sage-6.4
comment:6 Changed 7 years ago by
This gets really weird, actually.
sage: a.real() -1/2 sage: b.real() 1/2 sage: (a-b).real() # good -1 sage: (a-2*b).real() # good -3/2 sage: (a-3*b).real() # ho-hum -2 sage: (2*a-b).real() # yikes 1/2 sage: (3*a-b).real() # what? 3/4 sage: (4*a-b).real() # consistent... 1 sage: (5*a-b).real() # ?!?!? 5/4 sage: (-a-b).real() # seeing but not believing -1/4 sage: (-2*a-b).real() -1/2 sage: (-3*a-b).real() -3/4 sage: (2*a).real() # okay -1 sage: (2*a+b).real() # okay -1/2 sage: (3*a+b).real() -1 sage: (4*a+b).real() -3/2
This is consistent with Pynac somehow thinking that a.real()==1/4
when preceded by a positive constant and -1/4
when given a negative constant. But only when part of a sum including -b
, with +b
it's fine.
I tried to figure out what it was but so far no luck.
comment:7 Changed 6 years ago by
- Dependencies set to pynac-0.3.7
- Milestone changed from sage-6.4 to sage-6.7
- Report Upstream changed from N/A to Fixed upstream, in a later stable release.
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.
comment:8 Changed 6 years ago by
- Branch set to u/rws/taking_the_real_part_of_a_sum_of_exponentials_with_imaginary_exponents_gives_wrong_result
comment:9 Changed 6 years ago by
- Commit set to 410bab7e7016f06220e1a34335abbaa8f5e3e876
- Dependencies changed from pynac-0.3.7 to #18362
- Status changed from new to needs_review
comment:10 Changed 6 years ago by
- Reviewers set to Travis Scrimshaw
- Status changed from needs_review to positive_review
LGTM.
comment:11 Changed 6 years ago by
- Branch changed from u/rws/taking_the_real_part_of_a_sum_of_exponentials_with_imaginary_exponents_gives_wrong_result to 410bab7e7016f06220e1a34335abbaa8f5e3e876
- Resolution set to fixed
- Status changed from positive_review to closed
Thanks for the report! This is a bug in Pynac. I opened a ticket here:
http://hg.pynac.org/pynac/issue/15/wrong-result-from-real-part