* integrals
@ 2000-01-28 19:56 Peter Freyd
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From: Peter Freyd @ 2000-01-28 19:56 UTC (permalink / raw)
To: categories
Let I be a closed interval and let C(X) denote the bipointed
midpoint algebra of continuous functions from a space X to I.
I would not have guessed the following:
1.
Any map from C(X) to C(Y) that preserves midpoints, top, and
bottom is monotonic and preserves constant functions.
The order preservation is an immediate consequence of:
f =< g iff there exists h such that f|top = g|h
If we specialize to the case that X and Y are single points, the
order preservation implies the only map from I to I that preserves
midpoints, top and bottom, is the identity map (after first noticing
that it must have a dense subset of fixed points corresponding to the
dyadic rationals). Let I -> C(X) be the "K-map" that assigns
constant maps. For any y:Y let C(Y) -> I be its evaluation map.
Then for any map C(X) -> C(Y) that preserves midpoints, top, and
bottom we have that I -> C(X) -> C(Y) -> I is the identity map.
And from that we can easily conclude that C(X) -> C(Y) preserves
constant maps.
In a 22 Dec posting I said that the "mean-value" function M:C(I) -> I
can be characterized from its order preservation together with what it
does to constant functions and
MxM
C(I v I) --> C(I + I) --> C(I) x C(I) ---> I x I
C(F) | | m
v v
M
C(I) -----------------------------------> I
where F:I -> I v I is the coalgebra structure and m is the
midpoint operation. I went on to say that if C(F) is inverted then
this diagram can be read as a fixed-point definition of M: "It's the
unique fixed-point of an operator acting on the set of all those
order-preserving maps from C(I) to R that do the right thing on
constant functions." It's much better to change the R to I and say
just that it's acting on the set of bipointed midpoint-algebra
homomorphisms.
To recapitulate (using \ for lambda, T for top, B for bottom):
2.
The mean-value operator, M:C(I) --> I, is the unique map
such that:
M(\x.B) = B
M(\x.T) = T
(Mf)|(Mg) = M(\x.(fx)|(gx))
Mf = (M(\x.f(B|x)))|(M(\x.f(x|T)))
No inequalities, no limits, only equations.
Let me take the occasion to do a little clean-up. In a 31 Dec posting
I gave a curse-of-analysis proof for:
3.
Midpoint-preserving functions between intervals are monotonic.
Here's a much better proof. Monotonicity is equivalent to the
preservation of betweenness. Suppose f is a midpoint-preserving map
between intervals. It's conceptually useful to take the target to be,
instead, the real line and show that a failure to preserve betweenness
forces the values of f to be unbounded. So let a,b,c be points in
the source interval with b between a and c which fact will be
denoted as {a.b.c} and suppose that fb is not between fa and fc.
Without loss of generality we can assume that {fa.fc.fb}. There
exists {a.b'.c} such that either b = a|b' or b = c|b'. Each of
the equations fb = fa|fb' and fb = fc|fb' implies {fa.fc.fb'}.
So {a.b'.c} is still an example but the first equation implies that
the distance from fc to fb' is twice -- and the second equation
implies it is _at least_ twice -- the distance from fc to fb. By
iteration we may thus increase that distance beyond any bound.
A point that the previous proof didn't really cover is dispatched
by this corollary:
4.
Midpoint-preserving functions between intervals are either
one-to-one or constant.
Suppose a and c are distinct but fa = fc. If there's c' such
that c = a|c' then we may infer that fa = fc'. By replacing c
with c' in this manner as long as possible we may assume that a
and c are such that there is no solution to the equation c = a|x
and from that we may infer that if c' is chosen to be the endpoint
such that {a.c.c'} then there is {a,b,c} such that c = b|c'. By
the last result we have fa = fb = fc hence fc = fb = fc'. Thus we
may assume that a and c are distinct with fa = fc and c an
endpoint. Similarly we may assume that a is an endpoint and finish
by applying, once again, the last result.
Besides injectivity we have this corollary on surjectivity obtained
by combining 1 and 3:
5.
The image of any midpoint preserving function between intervals
is a closed interval.
PS
>From the proof of 1 it is clear that order preservation doesn't
require preservation of bottom. A similar proof uses preservation of
bottom instead of top, but if both conditions are dropped the map
needn't even be monotonic. Let I = [-1,1] and let C(I) -> I be
the map that sends f to f(-1)|(-f1). It sends all constant
functions to 0, it sends the identity map to -1 and it sends the
negating map to 1.)
The proof of 3 suggests that if the target interval is replaced by
the reals then there do exist midpoint-preserving maps that are not
monotonic. And that is, indeed, the case in the presence of the axiom
of choice: view the reals as a rational vector-space and count the
number of endomorphisms (each of which preserves midpoints) to find
that there are 2^{2^N} of them. But there are only 2^N monotonic
self maps (midpoint preserving or not).
Do we need the axiom of choice? If, instead, we add the axiom of
measurability, then the counterexamples disappear: let f be a
measurable midpoint-preserving map from R to R; we can easily
specialize to the case that f0 = 0, hence f is a measurable group
endomorphism; for any real a consider the induced group homomorphism
from R/aZ to R/(fa)Z; any measurable group character is continuous
and that's enough to force f to be continuous.
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