#15618: convert_map should not be a map
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Reporter: pbruin | Owner:
Type: defect | Status: new
Priority: major | Milestone: sage-6.2
Component: coercion | Resolution:
Keywords: conversion map | Merged in:
Authors: | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
Dependencies: | Stopgaps:
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Description changed by pbruin:
Old description:
> The following example shows that at least some convert_maps are
> incorrectly assumed to be homomorphisms:
> {{{
> sage: f = GF(11).convert_map_from(GF(7))
> sage: f
> Conversion map:
> From: Finite Field of size 7
> To: Finite Field of size 11
> sage: f.parent()
> Set of field embeddings from Finite Field of size 7 to Finite Field of
> size 11
> }}}
> This has the effect of constructing an element of a set that is in fact
> empty.
>
> Moreover, conversions should in general not be maps, because they are not
> necessarily defined everywhere.
>
> See also https://groups.google.com/forum/#!topic/sage-devel/Z4iNgVMFoms
New description:
The following examples show that some convert_maps are incorrectly assumed
to be homomorphisms:
{{{
sage: f = ZZ.convert_map_from(QQ)
sage: f.parent()
Set of Homomorphisms from Rational Field to Integer Ring
}}}
{{{
sage: f = ZZ.convert_map_from(GF(2))
sage: f.parent()
Set of field embeddings from Finite Field of size 2 to Integer Ring
}}}
{{{
sage: f = GF(11).convert_map_from(GF(7))
sage: f
Conversion map:
From: Finite Field of size 7
To: Finite Field of size 11
sage: f.parent()
Set of field embeddings from Finite Field of size 7 to Finite Field of
size 11
}}}
This has the effect of constructing an element of a set that is in fact
empty.
Moreover, conversions should in general only be assumed to be partial
maps, because they are not necessarily defined everywhere.
See also https://groups.google.com/forum/#!topic/sage-devel/Z4iNgVMFoms
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Ticket URL: <http://trac.sagemath.org/ticket/15618#comment:2>
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