# [FOM] characterization of Real numbers? (by Saeed Salehi)

Martin Davis martin at eipye.com
Thu Feb 10 15:14:47 EST 2005

```I'm posting this message for Saeed Salehi <saeed at cs.utu.fi> because his
attempts to post it contained unacceptable formatting codes.

Martin
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Hi everybody.

I tried to prove the below theorem posted by Prof. Friedman for answering
the second question of Prof. Shipman to myself, but came up with a counter
example. I might be wrong in some point, can anybody please tell me where I
am making a mistake?
Let X=R x [0,1] be the direct product of the reals with the closed interval
[0,1] equipped with the lexicographical order. It has the properties
mentioned in the theorem with the F function defined by F( (a,i) , (b,j) )
= ( (a+b)/2 , (i+j)/2 ). But X is not order isomorphic to R.
****************
That's a really nice theorem, which is just what I was looking for; just 2
questions:

1) what's the simplest way to formally define "order continuous" for a
function from X^2 to X where X is an ordered space?
2) How do you prove this?

-- JS

*****************
THEOREM. Let X be a linear ordering without endpoints. Then X is order
isomorphic to the real line if and only if

i) X has the least upper bound property;
ii) there is an order continuous F:X^2 into X such that for all x,y, x < y
implies x < F(x,y) < y.

Harvey Friedman

```