#16365: ChainComplex.betti uses wrong base_ring
----------------------------------+----------------------------
Reporter: dunfield | Owner:
Type: defect | Status: new
Priority: major | Milestone: sage-6.3
Component: algebraic topology | Keywords:
Merged in: | Authors:
Reviewers: | Report Upstream: N/A
Work issues: | Branch:
Commit: | Dependencies:
Stopgaps: |
----------------------------------+----------------------------
The docstring for `ChainComplex.betti` says
{{{
- ``base_ring`` -- a commutative ring (optional, default is the
base ring for the chain complex); compute homology with
these coefficients -- must be either the integers or a
field
}}}
however, in fact if `base_ring` is `None` then under the hood it defaults
to QQ. Thus the `betti` method will choke on examples where
`ChainComplex.homology` work fine:
{{{
sage: K = QuadraticField(-1, 'i')
sage: C = ChainComplex({0:matrix(K, [K.gen(),0])})
sage: C.homology()
{0: Vector space of dimension 1 over Number Field in i with defining
polynomial x^2 + 1,
1: Vector space of dimension 0 over Number Field in i with defining
polynomial x^2 + 1}
sage: C.betti()
[...huge traceback...]
TypeError: Unable to coerce i to a rational
}}}
and moreover gives **incorrect** answers when it silently coerces the
chain complex into QQ:
{{{
sage: C = ChainComplex({0:matrix(GF(5), [[3, 1],[1, 2]])})
sage: C.homology()
{0: Vector space of dimension 1 over Finite Field of size 5,
1: Vector space of dimension 1 over Finite Field of size 5}
sage: C.betti()
{0: 0, 1: 0}
}}}
I think the fix is just to change line 1344 of
`homology/chain_complex.py`. I will test this and upload a patch.
--
Ticket URL: <http://trac.sagemath.org/ticket/16365>
Sage <http://www.sagemath.org>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica,
and MATLAB
--
You received this message because you are subscribed to the Google Groups
"sage-trac" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
To post to this group, send email to [email protected].
Visit this group at http://groups.google.com/group/sage-trac.
For more options, visit https://groups.google.com/d/optout.
