Bug in Sage's monodromy pairing on elements of Brandt modules

[adeines]

In Magma:

Br:=BrandtModule(2,7);
g:=Br.1-Br.2;
InnerProduct(g,g);
6

In Sage:

Br = BrandtModule(2,7)
g1,g2 = Br.basis()
g = g1-g2
print g1,g2
print g.monodromy_pairing(g)
(1, 0) (0, 1)
2

Component: modular forms

Keywords: Brandt module, monodromy pairing

Author: Aly Deines, Gonzalo Tornaría

Branch: 448cd9d

Reviewer: Frédéric Chapoton

Issue created by migration from https://trac.sagemath.org/ticket/12866

[adeines]

Branch: u/aly.deines/ticket/12866

[tornaria]

Changed branch from u/aly.deines/ticket/12866 to u/tornaria/ticket/12866

[tornaria]

Commit: cede6e1

[tornaria]

comment:6

I rebased Aly's patch to sage-6.4.beta1. I added a series of commits to fix a typo in her patch, add some explanation of the computation that is being done, and then an asymptotic improvement so that the monodromy pairing can be computed in O(n) rather than O(n²) ideal computations.

---

New commits:

2f56770	Fixed Brandt module monodromy weights.
1891ad3	Fix Aly patch for Brandt module monodromy weights.
1fead1d	Brandt modules: extend _ideal_products() method
cede6e1	Trac 12866: improve asymptotic of monodromy_weights()

[tornaria]

Author: aly.deines, tornaria

[sagetrac-jakobkroeker]

Stopgaps: todo

[fchapoton]

Changed branch from u/tornaria/ticket/12866 to public/12866

[fchapoton]

Changed commit from cede6e1 to 8cb36d5

[fchapoton]

comment:9

I have made a branch with other changes to the file:

• many doc enhancements, and problems corrected
• use of cached_method instead of custom cache
• other small code changes.

---

New commits:

fcf022a	Merge branch 'u/tornaria/ticket/12866' into 7.1.rc0
8cb36d5	trac #12866 general spring cleanup of doc of Brandt modules

[fchapoton]

Changed author from aly.deines, tornaria to Aly Deines, Gonzalo Tornaria

[fchapoton]

Changed stopgaps from todo to none

[sagetrac-git]

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

448cd9d

Merge branch 'public/12866' into 7.3.rc0

[sagetrac-git]

Changed commit from 8cb36d5 to 448cd9d

[fchapoton]

Reviewer: Frédéric Chapoton

[fchapoton]

comment:15

I have checked the code of the authors.
My own changes are purely cosmetic, and I have double-checked them.
The bot has given a green light.

Therefore I am going to set this to positive review.

[vbraun]

Changed branch from public/12866 to 448cd9d

[fchapoton]

Changed author from Aly Deines, Gonzalo Tornaria to Aly Deines, Gonzalo Tornaría

[fchapoton]

Changed commit from 448cd9d to none