In number theory, Ostrowski's theorem, due to Alexander Ostrowski (1916), states that every non-trivial absolute value on the rational numbers Q is equivalent to either the usual real absolute value or a p-adic absolute value.
Contents
Definitions
Raising an absolute value to a power less than 1 always results in another absolute value. Two absolute values
The trivial absolute value on any field K is defined to be
The real absolute value on the rationals Q is the standard absolute value on the reals, defined to be
This is sometimes written with a subscript 1 instead of infinity.
For a prime number p, the p-adic absolute value on Q is defined as follows: any non-zero rational x, can be written uniquely as
Proof
Consider a non-trivial absolute value on the rationals
It suffices for us to consider the valuation of integers greater than one. For, if we find c in R+ for which
and for negative rationals
Case I: ∃n ∈ N |n|∗ > 1
Consider the following calculation. Let a, b and n be natural numbers with a, b > 1. Expressing bn in base a yields
Then we see, by the properties of an absolute value:
Therefore
However we have:
which implies:
Now choose 1 < b ∈ N such that |b|∗ > 1. Using this in the above ensures that |a|∗ > 1 regardless of the choice of a (else
i.e.
By symmetry, this inequality is an equality.
Since a, b were arbitrary, there is a constant,
Case II: ∀n ∈ N |n|∗ ≤ 1
As this valuation is non-trivial, there must be a natural number for which |n|∗ < 1. Factorising this natural,
yields |p|∗ must be less than 1, for at least one of the prime factors p = pj. We claim than in fact, that this is so for only one.
Suppose per contra that p, q are distinct primes with absolute value less than 1. First, let
a contradiction.
So we must have |pj|∗ = α < 1 for some j, and |pi|∗ = 1 for i ≠ j. Letting
we see that for general positive naturals
As per the above remarks we see that
One can also show a stronger conclusion, namely that
Another Ostrowski's theorem
Another theorem states that any field, complete with respect to an Archimedean absolute value, is (algebraically and topologically) isomorphic to either the real numbers or the complex numbers. This is sometimes also referred to as Ostrowski's theorem.