From: S.J.Vickers@open.ac.uk
To: categories@mta.ca
Subject: RE: Why binary products are ordered
Date: Tue, 30 Jan 2001 16:43:32 -0000 [thread overview]
Message-ID: <ACAB691EBFADD41196340008C7F35585C2BE87@tesla.open.ac.uk> (raw)
> One might say that the ordering of binary products, with a first
projection
> and a second projection, is spurious but inevitable.
>
> The two components of a binary product must be distinguished, as Colin
> McLarty explained, but they must be allowed to be isomorphic. The usual
> way we handle a situation like this in mathematics is to index them, in
> this case by a two-element set. ...
Two points.
First, even in computer programming, one can lose the need for an ordering
by using what are often called "record types", so that
<your age in years=99, your height in inches=70>
denotes the same record as
<your height in inches=70, your age in years=99>
I don't think there's anything mysterious about this. A pair of sets can be
described as a function f: X -> 2, and we can quite happily replace 2 by any
isomorphic set (but we don't have to choose the isomorphism) such as
M = {"your height in inches", "your age in years"}
The product of f: X -> M is then a universal solution to the problem of
finding
g: YxM -> X over M
and this is equivalent to the usual characterization once you have chosen
the isomorphism between M and 2.
A second, and deeper, point is that constructively there are unorderable
2-element sets, so there is a kind of binary product in which the ordering
first vs. second projection is impossible. It uses the same "record type"
construction.
An example in sheaves over the circle O is the twisted double cover M (edge
of a Mobius band). It is finite decidable set with cardinality 2. It is
isomorphic to 2 (i.e. 2xO) locally but not globally. It has no global
elements and no global total ordering. If you have a sheaf X with a map f: X
-> M, then locally it falls into two parts whose product you can take. It
can be expressed as
Pi f = {(i,x,j,y) in MxXxX | f(x) = i and f(y) = j and j = s(i)} / ~
where s: M -> M swaps the two elements and ~ is the equivalence relation
generated by (i,x,j,y) ~ (j,y,i,x).
Globally, Pi f is the equalizer of two maps from X^M to M^M, namely f^M and
the constant identity map: so set theoretically it is the set of sections,
{g: M -> X | g;f = Id_M}
The universal property is that for any YxM -> X over M you get a unique
corresponding Y -> Pi f.
The second description with X^M probably looks more natural to a topos
theorist, but the first one has the advantage of being geometric.
Steve.
next reply other threads:[~2001-01-30 16:43 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2001-01-30 16:43 S.J.Vickers [this message]
[not found] ` <20010131135719.A5824@kamiak.eecs.wsu.edu>
2001-02-01 11:10 ` S Vickers
-- strict thread matches above, loose matches on Subject: below --
2001-01-29 18:18 Charles Wells
2001-02-08 1:17 ` Vaughan Pratt
2001-02-08 9:14 ` Colin McLarty
2001-02-11 19:40 ` zdiskin
2001-02-08 17:44 ` Michael Barr
2001-02-11 1:54 ` zdiskin
2001-02-13 18:17 ` Nick Rossiter
2001-02-11 0:10 ` Dusko Pavlovic
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