* Does this topology have a name?
@ 2011-01-01 14:59 Michael Barr
2011-01-01 18:53 ` Dana Scott
[not found] ` <9CF079C4-EA66-4563-9DB0-FB3942D47DD9@cs.cmu.edu>
0 siblings, 2 replies; 3+ messages in thread
From: Michael Barr @ 2011-01-01 14:59 UTC (permalink / raw)
To: Categories list
Let A be a model of a finitary equational theory and let X be the set of
congruences on A. For a,b in A, let M(a,b) = {E} such that E is a
congruence on A and aEb. Does this topology have a name? It turns out
that this topology is coherent which means, among other things, that if we
make the M(a,b) clopen, the result is a Stone space.
Obviously in a ring, we could instead use the set of ideals, but aside
from the fact that that will include non-prime ideals, the topology is the
opposite of the Zariski topology.
Michael
[For admin and other information see: http://www.mta.ca/~cat-dist/ ]
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* Re: Does this topology have a name?
2011-01-01 14:59 Does this topology have a name? Michael Barr
@ 2011-01-01 18:53 ` Dana Scott
[not found] ` <9CF079C4-EA66-4563-9DB0-FB3942D47DD9@cs.cmu.edu>
1 sibling, 0 replies; 3+ messages in thread
From: Dana Scott @ 2011-01-01 18:53 UTC (permalink / raw)
To: Michael Barr; +Cc: Categories list
On Jan 1, 2011, at 6:59 AM, Michael Barr wrote:
> Let A be a model of a finitary equational theory and let X be the set of
> congruences on A. For a,b in A, let M(a,b) = {E} such that E is a
> congruence on A and aEb. Does this topology have a name? It turns out
> that this topology is coherent which means, among other things, that if we
> make the M(a,b) clopen, the result is a Stone space.
Consider the powerset space P(A x A) = 2^(A x A). The product topology
makes it a Stone space. This is elementary.
Now, the space X of congruences is defined by logical formulae with
only universal quantifiers and atomic formulae xEy for variables
ranging over A. That makes X a CLOSED subspace of P(A x A).
This is so easy, it hardly needs a name. And it works even if A has
infinitely many operations. That there is an equational theory in the
background seems neither here nor there to get a Stone space of congruences.
HAPPY NEW YEAR!
[For admin and other information see: http://www.mta.ca/~cat-dist/ ]
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* Re: Does this topology have a name?
[not found] ` <9CF079C4-EA66-4563-9DB0-FB3942D47DD9@cs.cmu.edu>
@ 2011-01-01 22:15 ` Michael Barr
0 siblings, 0 replies; 3+ messages in thread
From: Michael Barr @ 2011-01-01 22:15 UTC (permalink / raw)
To: Dana Scott; +Cc: Categories list
Yes, the fact that when these sets are taken as clopens gives a Stone
space is easy. But I want to know what to call the weaker topology in
which you take these sets as a basis of opens.
Happy New Year to you!
Michael
[For admin and other information see: http://www.mta.ca/~cat-dist/ ]
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2011-01-01 14:59 Does this topology have a name? Michael Barr
2011-01-01 18:53 ` Dana Scott
[not found] ` <9CF079C4-EA66-4563-9DB0-FB3942D47DD9@cs.cmu.edu>
2011-01-01 22:15 ` Michael Barr
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