SUO: Re: Relations And Their Divisitudes
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RATD. Note 11
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Tom,
Let us see if we can bring some visual intuitions --
the most fallible but still strangely useful kind --
into play. After all, this is where all that biz
about "projections" first came our our table talk.
Here is a picture of the initial corner of the 3-dimensional space
X = X_1 x X_2 x X_3 = !A!^3, where !A! = {a, b, c, ..., x, y, z},
as before, where I have plotted the point p = <a, b, c> in X and
further illustrated its projections on the three planes in view.
o-------------------------------------------------o
| |
| X_3 |
| |
| c |
| /|\ |
| / | \ |
| <a,c> . b \ |
| /|\ | \ |
| / | \| \ |
| / | \ . <b,c> |
| / | |\ /|\ |
| / | | \ / | \ |
| * = <a,b,c> o | o * | o |
| | | / \ | | | |
| | |/ \| | | |
| | a | | | |
| | / \ |\ | | |
| | / \ | \| | |
| | b \ | b | |
| | / \ | / \ | |
| |/ \|/ \| |
| c . c |
| X_1 \ <a,b> / X_2 |
| \ / |
| \ / |
| \ / |
| \ / |
| \ / |
| \ / |
| \ / |
| o |
| |
o-------------------------------------------------o
Figure 1. Plane Projections of the Point <a, b, c>
Reading off the Figure, we have this picture of the data:
With respect to the 1-adic projections of p = <a, b, c>:
p_1 (p) = p_1 <a, b, c> = a in X_1
p_2 (p) = p_2 <a, b, c> = b in X_2
p_3 (p) = p_3 <a, b, c> = c in X_3
With respect to the 2-adic projections of p = <a, b, c>:
p_12 (p) = p_12 <a, b, c> = <a, b> in X_1 x X_2
p_13 (p) = p_13 <a, b, c> = <a, c> in X_1 x X_3
p_23 (p) = p_23 <a, b, c> = <b, c> in X_2 x X_3
To finish up one of the questions about determination,
we can say that p = <a, b, c> is uniquely determined by
its 1-adic projections, simply because all that we really
mean by <a, b, c> is a thing that is uniquely determined by
its possessing the 1-adic projections a, b, c, respectively.
With this in mind, we have gained an edge on understanding
all that abstract nonsense in mathematical category theory
about "universal properties of diagrams". This is nothing
but a way of formalizing to the max the following insight:
If all we mean by <a, b, c> is a thing x that is uniquely
determined, within the appropriate context of discussion,
by the data p_1 (x) = a, p_2 (x) = b, p_3 (x) = c, then
anything else that is uniquely determined in that context
by that data is just <a, b, c>, whether we call it by any
other name, say, "ALPHABETSOUPSPOONFULL123", or anything
else, is really nothing other than <a, b, c> in disguise.
So let's watch out for that.
Jon Awbrey
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