commensurable numbers
Two positive real numbers a and b are commensurable, iff there exists a positive real number u such that
a=mu,b=nu | (1) |
with some positive integers m and n. If the positive numbers a and b are not commensurable, they are incommensurable.
Theorem. The positive numbers a and b are commensurable if and only if their ratio is a rational number
mn (m,n∈ℤ).
Proof. The equations (1) imply the proportion (http://planetmath.org/ProportionEquation)
ab=mn. | (2) |
Conversely, if (2) is valid with m,n∈ℤ, then we can write
a=m⋅bn,b=n⋅bn, |
which means that a and b are multiples of bn and thus commensurable. Q.E.D.
Example. The lengths of the side and the diagonal of http://planetmath.org/node/1086square are always incommensurable.
0.1 Commensurability as relation
-
•
The commensurability is an equivalence relation
in the set ℝ+ of the positive reals: the reflexivity
and the symmetry
are trivial; if a:b=r and b:c=s, then a:c=(a:b)(b:c)=rs, whence one obtains the transitivity.
-
•
The equivalence classes
of the commensurability are of the form
[ϱ]:= -
•
One of the equivalence classes is the set of the positive rationals, all others consist of positive irrational numbers.
-
•
If one sets , the equivalence classes form with respect to this binary operation
an Abelian group
.
Title | commensurable numbers |
Canonical name | CommensurableNumbers |
Date of creation | 2013-03-22 18:11:14 |
Last modified on | 2013-03-22 18:11:14 |
Owner | pahio (2872) |
Last modified by | pahio (2872) |
Numerical id | 13 |
Author | pahio (2872) |
Entry type | Definition |
Classification | msc 12D99 |
Classification | msc 03E02 |
Related topic | RationalAndIrrational |
Related topic | CommensurableSubgroups |
Defines | commensurable |
Defines | incommensurable |
Defines | commensurability |