Re: separable locale

Re: separable locale

[Eduardo J. Dubuc]

"separable" is used also to mean  T_2

Prof. Peter Johnstone wrote:
> Dear Thomas,
>
> I'm pretty sure that what Michael meant by "separable" was what
> most topologists would call "second countable" -- i.e., countably
> generated as a frame. (There are some topology textbooks in which
> this condition is called "completely separable".)
>
> Peter Johnstone
> ---------------------------------
> On Tue, 30 Jun 2009, Thomas Streicher wrote:
>
>> Recently rereading Fourman's "Continuous Truth" I came across the term
>> "separable locale" but could nowhere find an explanation. Does it mean a
>> cHa A for which there exists a countable subset B such that ever a in
>> A is
>> the supremum of those b in B with b leq a. This would be the point free
>> account of "second countable", i.e. having a countable basis.
>> Of course, second countable T_) spaces are separable, i.e. have a
>> countable
>> dense set.
>> Is this reading the "usual" one?
>>
>> Thomas

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Re: separable locale

[Michael Fourman]

> Prof. Peter Johnstone wrote:
>> Dear Thomas,
>>
>> I'm pretty sure that what Michael meant by "separable" was what
>> most topologists would call "second countable" -- i.e., countably
>> generated as a frame. (There are some topology textbooks in which
>> this condition is called "completely separable".)
>>
>> Peter Johnstone
>> ---------------------------------
>> On Tue, 30 Jun 2009, Thomas Streicher wrote:
>>
>>> Recently rereading Fourman's "Continuous Truth" I came across the term
>>> "separable locale" but could nowhere find an explanation. Does it mean a
>>> cHa A for which there exists a countable subset B such that ever a in
>>> A is
>>> the supremum of those b in B with b leq a. This would be the point free
>>> account of "second countable", i.e. having a countable basis.
>>> Of course, second countable T_) spaces are separable, i.e. have a
>>> countable
>>> dense set.
>>> Is this reading the "usual" one?
>>>
>>> Thomas
>

Thomas,

Peter is correct about my intention. More precisely, 'separable' is defined in
Formal Spaces (FS) (Fourman & Grayson, Brouwer Centenary Symposium) 3.12(c)
--- although I now find this account unnecessarily obscure.

What you say below is correct classically; constructively there is some subtlety.

A locale is separable iff it is presented (as in FS 1.1) by a countable language
with decidable \leq and countably many *inhabited* basic covers.


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Re: separable locale

[gcuri]

Dear Thomas,

as far as I remember, Fourman & Grayson define and study separable locales
toward the end of "Formal spaces".

With best regards

          Giovanni Curi

Quoting Thomas Streicher <streicher@mathematik.tu-darmstadt.de>:

> Recently rereading Fourman's "Continuous Truth" I came across the term
> "separable locale" but could nowhere find an explanation. Does it mean a
> cHa A for which there exists a countable subset B such that ever a in A is
> the supremum of those b in B with b leq a. This would be the point free
> account of "second countable", i.e. having a countable basis.
> Of course, second countable T_) spaces are separable, i.e. have a
> countable
> dense set.
> Is this reading the "usual" one?
>
> Thomas
>

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Re: separable locale

[Prof. Peter Johnstone]

Dear Thomas,

I'm pretty sure that what Michael meant by "separable" was what
most topologists would call "second countable" -- i.e., countably
generated as a frame. (There are some topology textbooks in which
this condition is called "completely separable".)

Peter Johnstone
---------------------------------
On Tue, 30 Jun 2009, Thomas Streicher wrote:

> Recently rereading Fourman's "Continuous Truth" I came across the term
> "separable locale" but could nowhere find an explanation. Does it mean a
> cHa A for which there exists a countable subset B such that ever a in A is
> the supremum of those b in B with b leq a. This would be the point free
> account of "second countable", i.e. having a countable basis.
> Of course, second countable T_) spaces are separable, i.e. have a
> countable
> dense set.
> Is this reading the "usual" one?
>
> Thomas
>


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separable locale

[Thomas Streicher]

Recently rereading Fourman's "Continuous Truth" I came across the term
"separable locale" but could nowhere find an explanation. Does it mean a
cHa A for which there exists a countable subset B such that ever a in A is
the supremum of those b in B with b leq a. This would be the point free
account of "second countable", i.e. having a countable basis.
Of course, second countable T_) spaces are separable, i.e. have a
countable
dense set.
Is this reading the "usual" one?

Thomas


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