Opened 9 years ago
Closed 5 years ago
#10629 closed defect (invalid)
performance of checking if (c/d)^(a/b) is rational
Reported by: | was | Owned by: | burcin |
---|---|---|---|
Priority: | minor | Milestone: | sage-duplicate/invalid/wontfix |
Component: | symbolics | Keywords: | |
Cc: | Merged in: | ||
Authors: | Reviewers: | Ralf Stephan, Karl-Dieter Crisman | |
Report Upstream: | N/A | Work issues: | |
Branch: | Commit: | ||
Dependencies: | Stopgaps: |
Description
sage: f(z) = (((-2)^z + 3^z)/((-2)^z - 3^z))^(1/z) sage: f(2) sqrt(-13/5) sage: f(10) (-60073/58025)^(1/10) sage: f(100) (-515377520732011332304111729993850674198810727377/515377520732011329768810529537391871205404316625)^(1/100) sage: f(100) (-515377520732011332304111729993850674198810727377/515377520732011329768810529537391871205404316625)^(1/100) sage: f(110) (-30432527221704537087670067466163877438919371148942073/30432527221704537085073919036896463624654122984332025)^(1/110) sage: f(120) (-1797010299914431210414509057505389955604379434598131450977/1797010299914431210411850601513820123858571820477570761825)^(1/120) sage: f(140) (-6265787482177970379256225588138505240370640792793057315382192174577/6265787482177970379256222800545355424042748100828273234337003927025)^(1/140) sage: f(160) (-21847450052839212624230656504451736779897953023116436713529106968318864898177/21847450052839212624230656501528733505236147186709067048096540929006999812225)^(1/160) sage: f(180) (-76177348045866392339289727720617094245965667291253555071028716054140756082155221621777/76177348045866392339289727720614029254883935513536838376974415435773518603910854417425)^(1/180)
... and these start getting INSANELY SLOW. Why?
(This was found by Vladimir V. Bondarenko.)
Change History (11)
comment:1 Changed 9 years ago by
comment:2 follow-up: ↓ 7 Changed 9 years ago by
Looking briefly over this, it's not surprising that it's factoring:
sage: (2^6 * 7^6)^(1/3) 196 sage: parent(_) Rational Field sage: (2^6 * 7^5)^(1/3) 28*49^(1/3) sage: parent(_) Symbolic Ring
So we need to factor to decide if the result is in the rationals or not.
comment:3 Changed 6 years ago by
- Milestone changed from sage-5.11 to sage-5.12
comment:4 Changed 6 years ago by
- Milestone changed from sage-6.1 to sage-6.2
comment:5 Changed 6 years ago by
- Milestone changed from sage-6.2 to sage-6.3
comment:6 Changed 5 years ago by
- Milestone changed from sage-6.3 to sage-6.4
comment:7 in reply to: ↑ 2 Changed 5 years ago by
Replying to ddrake:
So we need to factor to decide if the result is in the rationals or not.
Can someone explain this further? I mean technically, factoring is overkill for this---if the exponent is a rational a/b
then it suffices to check if the base is a b-th power, doesn't it?
comment:8 Changed 5 years ago by
- Summary changed from weird performance issue -- why is this SOOOO slow !? to performance of checking if (c/d)^(a/b) is rational
comment:9 Changed 5 years ago by
- Milestone changed from sage-6.4 to sage-duplicate/invalid/wontfix
- Status changed from new to needs_review
Ah ok, it's not so important to decide whether rational or not, but to get the "rational power parts" in order to get a simplified expression of form x*y^(a/b)
. Since there is no way out of factoring I propose to close the ticket.
comment:10 Changed 5 years ago by
- Reviewers set to Ralf Stephan, Karl-Dieter Crisman
- Status changed from needs_review to positive_review
Hmm, but one could possibly ask for it not to do that simplified form, to just always go straight to the symbolic ring... but in this case I agree that unless William really complains, we can close this.
comment:11 Changed 5 years ago by
- Resolution set to invalid
- Status changed from positive_review to closed
Note the traceback:
so it is factoring.