# [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...

#################################

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

```