From: Andrej Bauer <Andrej.Bauer@andrej.com>
To: Category Mailing List <categories@mta.ca>
Subject: Re: Limits
Date: 02 May 2001 19:10:56 +0200 [thread overview]
Message-ID: <vkaoftb3cdb.fsf@laurie.pc.cs.cmu.edu> (raw)
In-Reply-To: Tobias Schroeder's message of "Wed, 2 May 2001 15:04:00 +0200 (CEST)"
Tobias Schroeder <tschroed@Mathematik.Uni-Marburg.de> writes:
> So I'd be very grateful for answers to one of the following:
> - Can the limit of a sequence of real numbers be expressed
> as a categorical limit (of course it can if the sequence is
> monotone, but what if it is not)?
With a little bit of cheating, you can use domain theory to express
the limit as a sequence as a _colimit_ in a partially ordered set.
Let D be the partial order consisting of all the closed intervals,
including singletons [a,a], ordered by reverse inclusion. We can
embed R into D by mapping it to the maximal elements a |---> [a,a],
and under a suitable topology on D (the Scott topology), this is
a topological embedding--purists may want to throw in R as the
smallest element to obtain an honest continuous domain.
Let x_i be a Cauchy sequence of real numbers. To say that x_i is a
Cauchy sequence is to say that there exist numbers d_i such that
(1) For j >= i, the interval [x_i - d_i, x_i + d_i]
contains [x_j + d_j, x_j + d_j].
(2) The numbers d_i become arbitrarily small: for every k
there is i such that for all j >= i, d_i < 1/k.
(Exercise for your students: show that this is equivalent to the usual
definition of Cauchy sequence.)
In terms of the partial order D, (1) says that the intervals
[x_i - d_i, x_i + d_i] form an increasing sequence. Every increasing
sequence in D has a supremum, because an intersection of a nested
sequence of closed intervals is a closed interval, so let
[u,v] = sup_i [x_i - d_i, x_i + d_i]
By (2), we get that u = v, and we have obtained the limit of the
sequence (x_i) as a supremum. Supremums are the _colimits_ in a
partial order. If you prefer limits, you can stand on your head.
I do not see how to get by without using the _evidence_ that (x_i) is
a Cauchy sequence, i.e., the numbers d_i. This is intuitionistic
mathematics creeping in, which is just as well.
> - Why have people chosen the term "limit" in category theory?
> (And, by the way, who has defined it first?)
I am way too young to know the answer to this.
Andrej
next prev parent reply other threads:[~2001-05-02 17:10 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2001-05-02 13:04 Limits Tobias Schroeder
2001-05-02 17:10 ` Andrej Bauer [this message]
2001-05-03 12:59 ` Limits Martin Escardo
2001-05-03 23:15 ` Limits Dusko Pavlovic
2001-05-02 17:02 Limits Peter Freyd
2001-05-05 18:58 ` Limits jim stasheff
2001-05-03 23:38 Limits jdolan
2001-05-10 2:18 ` Limits Dusko Pavlovic
2001-05-04 21:04 Limits jdolan
2001-05-06 0:26 ` Limits Ross Street
2001-05-16 22:46 Limits Paul H Palmquist
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=vkaoftb3cdb.fsf@laurie.pc.cs.cmu.edu \
--to=andrej.bauer@andrej.com \
--cc=categories@mta.ca \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).