Opened 2 years ago
Closed 8 months ago
#30749 closed defect (fixed)
Characteristic polynomial of central Hyperplane arrangement returns wrong result?
Reported by:  JeanPhilippe Labbé  Owned by:  

Priority:  major  Milestone:  sage9.6 
Component:  geometry  Keywords:  hyperplane arrangements, regions 
Cc:  ghkliem, Laith Rastanawi, Julian Ritter  Merged in:  
Authors:  Jonathan Kliem  Reviewers:  Travis Scrimshaw 
Report Upstream:  N/A  Work issues:  
Branch:  d74929d (Commits, GitHub, GitLab)  Commit:  d74929dfb74820f79d574189a6bc5803dc00dd8d 
Dependencies:  Stopgaps: 
Description (last modified by )
A central hyperplane arrangement must have an even number of regions by central symmetry... yet the one below gets 31 regions(!).
R.<y> = QQ[] v1 = AA.polynomial_root(AA.common_polynomial(y^2  3), RIF(RR(1.7320508075688772), RR(1.7320508075688774))) v2 = QQbar.polynomial_root(AA.common_polynomial(y^4  y^2 + 1), CIF(RIF(RR(0.8660254037844386), RR(0.86602540378443871)), RIF(RR(0.50000000000000011), RR(0.49999999999999994)))) my_vectors = (vector(AA, [v1, 1, 1]), vector(AA, [0, 2, 1]), vector(AA,[v1, 1, 1]), vector(AA, [1, 0, 0]), vector(AA, [1/2, AA(1/2*v2^3 + v2),0]), vector(AA, [1/2, AA(1/2*v2^3 + v2), 0])) sage: H = HyperplaneArrangements(AA,names='xyz') sage: x,y,z = H.gens() sage: A = H(backend="normaliz") sage: for v in my_vectors: ....: a,b,c = v ....: A = A.add_hyperplane(a*x + b*y + c*z) sage: A Arrangement of 6 hyperplanes of dimension 3 and rank 3 sage: A.n_regions() 31 sage: A.is_central() True
Here is another failure in characteristic polynomial:
sage: tau = AA((1+sqrt(5))/2) sage: ncn = [[2*tau+1,2*tau,tau],[2*tau+2,2*tau+1,tau+1],[1,1,1],[tau+1,tau+1,tau],[2*tau,2*tau,tau],[tau+1,tau+1,1],[1,1,0],[0,1,0],[1,0,0],[tau+1,tau,tau]] sage: H = HyperplaneArrangements(AA,names='xyz') sage: x,y,z = H.gens() sage: A = H() sage: for v in ncn: ....: a,b,c = v ....: A = A.add_hyperplane(a*x + b*y + c*z) ....: sage: A.n_regions() Traceback (most recent call last): ... ValueError: arrangement cannot simultaneously have h and h as hyperplane
#30078 fixes this and we add another doctest here.
Change History (18)
comment:1 Changed 2 years ago by
Description:  modified (diff) 

comment:2 followup: 3 Changed 2 years ago by
comment:3 Changed 2 years ago by
Replying to chapoton:
The char poly gives the same answer
sage: p = A.characteristic_polynomial() : p x^3  6*x^2 + 13*x  11 sage: p(1) 31
Yes, this is how the number of regions is computed up to sign. The hyperplane arrangement seems to be sane: computing .regions()
computes the expected (number of) regions.
I asked the number of regions to doublecheck something and then got this scary answer...
comment:4 Changed 2 years ago by
Summary:  Characteristic polynomial of central Hyperplane arrangement results wrong result? → Characteristic polynomial of central Hyperplane arrangement returns wrong result? 

comment:5 followup: 10 Changed 2 years ago by
I encountered this problem in some example a long while ago, before I was contributing to Sage myself. A simple workaround is to have n_regions
output len(self.regions())
, which was good enough for me. Of course, this is much slower if one doesn’t want to compute the regions anyway. I don’t understand the characteristic polynomial well enough to figure out the error in there.
comment:6 Changed 2 years ago by
Description:  modified (diff) 

comment:7 Changed 22 months ago by
Milestone:  sage9.3 → sage9.4 

Setting new milestone based on a cursory review of ticket status, priority, and last modification date.
comment:8 Changed 16 months ago by
Milestone:  sage9.4 → sage9.5 

comment:9 Changed 12 months ago by
Milestone:  sage9.5 → sage9.6 

comment:10 Changed 9 months ago by
Replying to nailuj:
I don’t understand the characteristic polynomial well enough to figure out the error in there.
The characteristic polynomial is obtained through recursive deletions and contractions of hyperplane arrangements. During this process, it may occur that some hyperplane h
is included in an arrangement multiple times with different scalings of the defining linear expression which are not properly detected as duplicates. This is due to a defect in the method hyperplane.primitive()
addressed in #30078.
comment:11 Changed 9 months ago by
Dependencies:  → #30078 

comment:13 Changed 8 months ago by
It seems like it is. Using the first example, I get
sage: A.n_regions() 24 sage: len(A.regions()) 24 sage: A.characteristic_polynomial()(1) 24
So it is now consistent. We probably just want to add a doctest for this ticket.
comment:14 Changed 8 months ago by
Authors:  → Jonathan Kliem 

Branch:  → public/30749 
Commit:  → 1492ed150e76f36d09317428d9e642986554ec6b 
Dependencies:  #30078 
Description:  modified (diff) 
Status:  new → needs_review 
comment:15 Changed 8 months ago by

sage/geometry/hyperplane_arrangement/arrangement.py
a b 1179 1179 sage: H = HyperplaneArrangements(AA, names='xyz') 1180 1180 sage: x,y,z = H.gens() 1181 1181 sage: A = H(backend="normaliz") # optional  pynormaliz 1182 sage: for v in my_vector : # optional  pyrormaliz1182 sage: for v in my_vectors: # optional  pynormaliz 1183 1183 ....: a, b, c = v 1184 1184 ....: A = A.add_hyperplane(a*x + b*y + c*z) 1185 sage: A.n_regions() # optional  py rormaliz1185 sage: A.n_regions() # optional  pynormaliz 1186 1186 32 1187 1187 """ 1188 1188 if self.base_ring().characteristic() != 0:
comment:16 Changed 8 months ago by
Commit:  1492ed150e76f36d09317428d9e642986554ec6b → d74929dfb74820f79d574189a6bc5803dc00dd8d 

Branch pushed to git repo; I updated commit sha1. New commits:
d74929d  typos

comment:17 Changed 8 months ago by
Reviewers:  → Travis Scrimshaw 

Status:  needs_review → positive_review 
Thank you. LGTM.
@ghsheerluck, please add your (real) name to the reviewers (if you want).
comment:18 Changed 8 months ago by
Branch:  public/30749 → d74929dfb74820f79d574189a6bc5803dc00dd8d 

Resolution:  → fixed 
Status:  positive_review → closed 
The char poly gives the same answer