Opened 5 years ago
Closed 5 years ago
#16240 closed defect (fixed)
regression in partial_fraction_decomposition()
Reported by:  rws  Owned by:  

Priority:  major  Milestone:  sage6.3 
Component:  commutative algebra  Keywords:  partial fractions, polynomials 
Cc:  robertwb  Merged in:  
Authors:  Ralf Stephan  Reviewers:  Peter Bruin 
Report Upstream:  N/A  Work issues:  
Branch:  41fbef4 (Commits)  Commit:  41fbef419f6f3d3681eefd4a8f302bd1504effe2 
Dependencies:  Stopgaps: 
Description (last modified by )
This worked in 6.1.1. Maybe triggered in #15306. Note the sign of the denominator:
sage: R.<x> = ZZ['x'] sage: p=(6*x^2  9*x + 5)/(x^3 + 3*x^2  3*x + 1) sage: p.partial_fraction_decomposition() (0, [6/(x  1), 3/(x^2  2*x + 1), 2/(x^3  3*x^2 + 3*x  1)]) sage: 6/(x  1) + 3/(x^2  2*x + 1) + 2/(x^3  3*x^2 + 3*x  1) (6*x^2  9*x + 5)/(x^3  3*x^2 + 3*x  1)
while in SR:
sage: var('x') x sage: p=(6*x^2  9*x + 5)/(x^3 + 3*x^2  3*x + 1) sage: p.partial_fraction() 6/(x  1)  3/(x  1)^2  2/(x  1)^3
The minimal case, showing with odd exponents:
sage: R.<x> = ZZ['x'] sage: p=1/(x + 1) sage: p.partial_fraction_decomposition() (0, [1/(x  1)])
Change History (9)
comment:1 Changed 5 years ago by
 Description modified (diff)
 Priority changed from critical to major
comment:2 Changed 5 years ago by
 Description modified (diff)
 Summary changed from plain wrong results from partial_fraction_decomposition() to regression in partial_fraction_decomposition()
comment:3 Changed 5 years ago by
comment:4 Changed 5 years ago by
 Branch set to u/rws/regression_in_partial_fraction_decomposition__
comment:5 Changed 5 years ago by
 Commit set to 41fbef419f6f3d3681eefd4a8f302bd1504effe2
 Status changed from new to needs_review
New commits:
41fbef4  16240: fix sign of numerator when factor() scrambles denominators

comment:6 Changed 5 years ago by
 Reviewers set to Peter Bruin
OK, I convinced myself that if the algorithm is correct in the case where the unit in the factorisation of the denominator is 1 (in which case this patch does not make a difference), then the algorithm is correct for arbitrary units if and only if the patch is applied.
As Knuth said, "I have only proved it correct, not tried it", so now running doctests.
comment:7 Changed 5 years ago by
 Status changed from needs_review to positive_review
comment:8 Changed 5 years ago by
comment:9 Changed 5 years ago by
 Branch changed from u/rws/regression_in_partial_fraction_decomposition__ to 41fbef419f6f3d3681eefd4a8f302bd1504effe2
 Resolution set to fixed
 Status changed from positive_review to closed
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