Virgil
Posts:
9,012
Registered:
1/6/11


Re: What is the intuitive meaning of "nonArchimedean" for a valued field?
Posted:
May 13, 2013 11:25 PM


In article <Pine.NEB.4.64.1305131912300.1051@panix2.panix.com>, William Elliot <marsh@panix.com> wrote:
> On Mon, 13 May 2013, mike3 wrote: > > > I'm curious about this. The "Archimedean property" for an _ordered_ > > field F means that given any positive elements a and b in F, with a < > > b, then there exists a natural number n such that na < b. > > That is trival. If a < b, then 1a < b and the Archimedean property > always holds. > > > Intuitively, this means F has no "infinitely big" or "infinitely small" > > elements. > > > We could also say that "given any positive element a, then > > there is a natural number n such that na > 1". If the property fails, > > then F contains "infinitely small" elements. > > > What does infinitely large and infinitely small mean?
In this case that if a is positive and "infinitely small" and b is positive but not "infinitely small" then for every natural number n, na < b. ANd similarly if a is positive but not "infinitely large" and b is both positive and "infinitely large" then for every natural number n, na < b. > > > Now, there is an analogous property for nonordered, "valued" fields > > (fields with an "absolute value" function added). The "Archimedean > > property" here means that given any nonzero element a e F, that there > > exists a natural number n such that na > 1. But what, intuitively, > > does it mean when this property fails? > > That there's some element a, for which the sequence > (na)_n is bounded by 1. > > > In that case, there aren't any elements with "infinitely small but > > nonzero" absolute value since the absolute value functions are usually > > taken as realvalued, and the reals are Archimedean (as an ordered > > field). Instead, what happens for such a realvalued absolute value is > > that the triangle inequality strengthens to a + b <= max(a, b) and > > not just a + b <= a +  b. This causes the space to behave really > > weirdly(*). But what is the _intuition_ here, and how does this notion > > relate, if at all, to the first one? Especially considering I see in > > papers like this: > > It seems such a norm would give rise to an ultra metric. 

