Opened 6 years ago
Last modified 6 years ago
#19830 new defect
Classification of finite and affine Coxeter groups
Reported by:  stumpc5  Owned by:  

Priority:  major  Milestone:  sage7.1 
Component:  combinatorics  Keywords:  coxeter system, root system 
Cc:  tscrim, chapoton, nthiery, darij  Merged in:  
Authors:  Travis Scrimshaw  Reviewers:  
Report Upstream:  N/A  Work issues:  
Branch:  public/combinat/fix_coxeter_type_relabelling19830 (Commits, GitHub, GitLab)  Commit:  d86d0abf581635109f147bc3b75c7efb0d819899 
Dependencies:  Stopgaps: 
Description
The bug here is twofold:
 The roots for a
CoxeterGroup
are constructed as in Humphreys "in the Coxeter sense, that all have the same norm". Therefore it seems more natural to not at all introduce a type C and to follow Humphrey's classification of finite Coxeter groups given on page 32.
 The current implementation of types B/C (which I suggest to remove and to only keep type B) is also broken:
 First Sage start
sage: W = CoxeterGroup(['B',4]) sage: W.coxeter_matrix().coxeter_type() Coxeter type of ['B', 4] sage: W = CoxeterGroup(['C',4]) sage: W.coxeter_matrix().coxeter_type() Coxeter type of ['B', 4]
 Second Sage start
sage: W = CoxeterGroup(['C',4]) sage: W.coxeter_matrix().coxeter_type() Coxeter type of ['C', 4] sage: W = CoxeterGroup(['B',4]) sage: W.coxeter_matrix().coxeter_type() Coxeter type of ['C', 4]
Change History (16)
comment:1 Changed 6 years ago by
comment:2 Changed 6 years ago by
With 7.0.beta1
, I get:
sage: CoxeterGroup(CoxeterType(['B',2],['B',2]).coxeter_matrix()).coxeter_matrix().coxeter_type() Coxeter type of B2xB2
What are you using when you tested that? Although, that you got that in any situation is very frightening to me.
comment:3 Changed 6 years ago by
Hm, after restarting with 7.0.beta2
I also get this result, and I wasn't able to reproduce the bug even after retyping all comments that seemed to be related. So let's leave this aside for now.
comment:4 Changed 6 years ago by
At some point during that session I accidentally copied almost the complete source of subword_complex.py
into the terminal. So it might also be that this caused the issue, though I wouldn't know how.)
comment:5 Changed 6 years ago by
@tscrim: And what to do with
sage: sage: W = CoxeterGroup(['B',3],index_set=['A','b','XX']) sage: sage: W.index_set() (1, 2, 3)
comment:6 followup: ↓ 10 Changed 6 years ago by
Sorry for adding more and more questions here: The documentation of the method roots
states
These are roots in the Coxeter sense, that all have the same norm. They are given by their coefficients in the base of simple roots, also taken to have all the same norm.
How is this true if the sum of the coefficients is not one? As in
sage: W = CoxeterGroup(['A',2]) sage: W.roots() [(1, 0), (1, 1), (0, 1), (1, 0), (1, 1), (0, 1)]
comment:7 Changed 6 years ago by
What ? Type A2 is simply laced, so the roots are just the roots in the Weyl sense..
The scalar product is the one that is invariant by the group, of course..
comment:8 Changed 6 years ago by
I am working on trying to fix the norm index set issue, which uncovered some other bugs/isues with the relabelling. I was somewhat inclined to say that passing a Coxeter (or Cartan) type and an index set was essentially double info, but after some more thought, I think it is a nice API to support.
comment:9 Changed 6 years ago by
@chapoton: My last comment on the root lengths was too quick, please forget about it and sorry for the noise!
comment:10 in reply to: ↑ 6 Changed 6 years ago by
Replying to stumpc5:
Sorry for adding more and more questions here: The documentation of the method
roots
statesThese are roots in the Coxeter sense, that all have the same norm. They are given by their coefficients in the base of simple roots, also taken to have all the same norm.How is this true if the sum of the coefficients is not one? As in
sage: W = CoxeterGroup(['A',2]) sage: W.roots() [(1, 0), (1, 1), (0, 1), (1, 0), (1, 1), (0, 1)]
Isn't this just that the basis of the simple roots is not orthonormal? So one can't read of the norm of vectors expressed there right away.
comment:11 Changed 6 years ago by
Oops, I was too quick and did not see your last comment :)
comment:12 Changed 6 years ago by
 Branch set to public/combinat/fix_coxeter_type_relabelling19830
 Cc darij added
 Commit set to d86d0abf581635109f147bc3b75c7efb0d819899
 Milestone changed from sage7.0 to sage7.1
Here is a fix for relabelling issues that I came across. Although I think this might be better on a separate ticket.
@darij the previous test for checking for relabelled types gave false negatives. Consider F_{4} under cyclic shift and under interchanging 1 and 3. These are the same type and our relabelling check does not distinguish between them (nor can any algorithm).
How do we want to handle Coxeter vs Dynkin classification of the affine types or the general case? I'm guessing for the affine types, we just use the untwisted types. Do we even want to consider the general case at this point?
New commits:
d86d0ab  Improving support for relabelled Coxeter types.

comment:13 followup: ↓ 14 Changed 6 years ago by
Oh gosh, I have literally no idea about anything beyond the classical types. Is this a bug in the algorithm I wrote? I thought very little of that algorithm is still in Sage?
comment:14 in reply to: ↑ 13 Changed 6 years ago by
Replying to darij:
Oh gosh, I have literally no idea about anything beyond the classical types. Is this a bug in the algorithm I wrote? I thought very little of that algorithm is still in Sage?
Sorry, I meant the B/C issue in the ticket description.
comment:15 Changed 6 years ago by
Oh, that!
I have to admit I don't know what the current recognition code is doing (the pathdependent output suggests some caching is going on), but my original code in #16630 never outputted a C_n. Maybe we should do the same?
comment:16 Changed 6 years ago by
The B/C issue isn't so much having to do with type recognition (which won't result in a C_{n}), but instead what to do when we do things like CoxeterType(['C', 5])
.
And what to do with