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Topic: This Week's Finds in Mathematical Physics (Week 240)
Replies: 3   Last Post: Oct 27, 2006 10:00 AM

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john baez

Posts: 460
Registered: 12/6/04
Re: This Week's Finds in Mathematical Physics (Week 240)
Posted: Oct 26, 2006 12:14 PM
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In article <ehmgfb$mdr$1@news.ks.uiuc.edu>,
Esa A E Peuha <esa.peuha@helsinki.fi> wrote:

>baez@math.removethis.ucr.andthis.edu (John Baez) writes:

>> In analysis, X^Y
>> can approach anything between 0 and 1 when X and Y approach 0.


>If X^Y approaches Z, then X^(-Y) appoaches 1/Z, so the actual range of
>possible values is between 0 and infinity, including both endpoints.


I was imagining both X and Y being positive, perhaps because I had
set theory on my mind, and sets have a nonnegative number of elements. :-)
But, of you're right if Y can be negative.

Here are some other corrections and comments, which have been
used to improve the web version of "week240", available here:

http://math.ucr.edu/home/baez/week240.html

When I wrote the first version of this Week's Finds, I was ignorant of work
before Dolan and Trimble's that also described the free cartesian closed
category on one object in terms of games. I was gently set straight by
Dominic Hughes, who has permitted me to attach this post of his from the
category theory mailing list:

This "backtracking game" characterisation has been known since around
'93-'94, in the work of Hyland and Ong:

M. Hyland and L. Ong. On full abstraction for PCF. Information and
Computation, Volume 163, pp. 285-408, December 2000. [Under review for
6 years!]
ftp://ftp.comlab.ox.ac.uk/pub/Documents/techpapers/Luke.Ong/pcf.ps.gz

(PCF is an extension of typed lambda calculus.) My D.Phil. thesis
extended the lambda calculus (free CCC) characterisation to second-order,
published in:

Games and Definability for System F. Logic in Computer Science, 1997
http://boole.stanford.edu/~dominic/papers/

To characterise the free CCC on an *arbitrary* set {Z,Y,X,...} of
generators (rather than a single generator, as you discuss), one simply
adds the following Copycat Condition:

(*) Whenever first player plays an occurrence of X, the second player
must play an occurrence of X.

[Try it: see how X -> Y -> X has just one winning strategy.] Although the
LICS'97 paper cited above appears to be the first place the Copycat
Condition appears in print, I like to think it was already understood at
the time by people working in the area. Technically speaking, winning
strategies correspond to eta-expanded beta-normal forms. See pages 5-7 of
my thesis for an informal description of the correspondence.

It sounds like you've reached the point of trying to figure out how
composition should work. Proving associativity is fiddly. Hyland and Ong
give a very elegant treatment, via a larger CCC of games in which *both*
players can backtrack. The free CCC subcategory is carved out as the
so-called *innocent* strategies. This composition is almost identical to
that presented by Coquand in:

A semantics of evidence for classical arithmetic. Thierry Coquand.
Proceedings of the CLICS workshop, Aarhus, 1992.

Dominic

PS A game-theoretic characterisation with an entirely different flavour
(winning strategies less "obviously" corresponding to eta-long beta-normal
forms) is:

Abramsky, S., Jagadeesan, R. and Malacaria, P., Full Abstraction for
PCF. Info. & Comp. 163 (2000), 409-470.
http://web.comlab.ox.ac.uk/oucl/work/samson.abramsky/pcf.pdf
[Announced concurrently with Hyland-Ong, around '93-'94.]

On a different subject, James Dolan had this to say:

you describe holodeck strategies for "lady or tiger" where you take
back "just when the tiger is about to eat you", but that's not the
way it works. you take back just _after_ the tiger has eaten you.

(i guess that this is partially because of your lack of experience
with computer games with a "saved game" feature. typically you die
in the game and the computer plays some sort of funeral or at least
funereal music; then you're taken to the reincarnation gallery where
you select one to return to from your catalog of previous lives.
or something like that.)

Also, in the first version of this Week's Finds I claimed that all systematic
ways of picking an element of (X^X)^(X^X) could be defined using the
lambda calculus. I was disabused of this notion by Vaughan Pratt, who
wrote:

Hi, John,

In "week240", you said

The moral of this game is that all systematic methods for picking
an element of (X^X)^(X^X) for an unknown set X can be written
using the lambda calculus.

What is unsystematic about the contagious-fixpoint functional?
This is the functional that maps those functions that have any
fixpoints to the identity function (the function that makes
every element a fixpoint) and functions without fixpoints to
themselves (thus preserving the absence of fixpoints). It's a
perfectly good functional that is equally well defined for all
sets X, its statement in no way depends on X, and conceptually
the concept of contagious fixpoints is even intuitively natural,
but how do you write it using the lambda calculus?

Many more examples in this vein at JPAA 128, 33-92 (Pare and Roman,
Dinatural numbers, 1998). The above is the case K = {0} of Freyd's
(proper) class of examples.

Vaughan

Here Pratt uses "functional" to mean what I was calling an "operator".

Finally, Tom Payne at UCR coaxed me into using more standard lambda
calculus notation.

.........................................................................

Puzzle 29: Stravinsky's Rite of Spring caused a ruckus in Paris, but
which piece actually got him *arrested* in Boston?

If you get stuck, try:

http://math.ucr.edu/home/baez/puzzles/





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