[FOM] Characterization of Linear Continua
Harvey Friedman
friedman at math.ohio-state.edu
Wed Feb 4 11:07:09 EST 2004
This will be put into the numbered postings (hopefully with further
results/questions) if it survives claims by others...
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A linear continuum is a linear ordering which
i) has at least two points;
ii) is dense (between any two points there is a third);
iii) every nonempty subset which is bounded above has a least upper bound;
iv) every nonempty subset which is bounded below has a greatest lower bound.
WARNING: This terminology, linear continuum, may not be standard.
Note that a linear continuum may or may not have endpoints.
Let X be a linear continuum. It is obvious that we have a robust notion of
continuous functions f:X into X, using open intervals surrounding points. It
is also obvious that we have a robust notion of continuous f:X dot X into X.
THEROEM. Let X,Y be linear continua both satisfying the following condition.
There is a continuous (everywhere defined) binary function f such that for
all x < y, x < f(x,y) < y. Then X,Y are isomorphic, if we remove the
endpoints (if any). In fact, X,Y with endpoints removed are isomorphic to
the real line.
We can view this as a
*derivation of separability*.
Harvey Friedman
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