Difference between revisions of "Uncountable"
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− | {{ | + | A [[set]] <math>S</math> is said to be '''uncountable''' if there is no [[injection]] <math>f:S\to\mathbb{N}</math>. Assuming the [[Axiom of choice]], every set that is ''not'' uncountable is either [[finite]] or [[countably infinite]]. The most common example of an uncountable set is the set of [[real number]]s <math>\mathbb{R}</math>. |
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+ | == Proof that <math>\mathbb{R}</math> is uncountable == | ||
− | + | We give an [[indirect proof]] here. This is one of the most famous indirect proofs and was first given by [[Georg Cantor]]. | |
− | + | Suppose that the set <math>A=\{x\in\mathbb{R}:0<x< 1\}</math> is countable. Let <math>\{\omega_1, \omega_2, \omega_3, ...\}</math> be any enumeration of the elements of <math>A</math> (in other words, take an injection <math>f: A \to \mathbb{N}</math>, and denote <math>\omega_i = f(i)</math>). | |
− | + | Consider the [[decimal expansion]] of each <math>\omega_i</math>, say <math>\omega_i=0.b_{i1}b_{i2}b_{i3} \ldots</math> for all <math>i</math>. Now construct a real number <math>\omega= 0.b_1b_2b_3 \ldots</math>, by choosing the digit <math>b_i</math> so that it differs from <math>b_{ii}</math> by at least 3 and so that <math>b_i</math> is never equal to 9 or 0. It follows that <math>\omega</math> differs from <math>\omega_i</math> by at least <math>\frac{2}{10^i}</math>, so <math>\omega \neq \omega_i</math> for every <math>i</math>. Thus, <math>\omega \not \in A</math>. However, <math>\omega</math> is clearly a real number between 0 and 1, a [[contradiction]]. Thus our assumption that <math>A</math> is countable must be false, and since <math>\mathbb{R} \supset A</math> we have that <math>\mathbb{R}</math> is uncountable. | |
− | + | An alternative proof uses [[Cantor's Theorem]], which says that for all sets <math>S</math>, we have <math>|S|<|\mathcal{P}(S)|</math>, where <math>\mathcal{P}(S)</math> is the [[power set]] of <math>S</math>. First, we note that the [[Cantor set]] <math>\mathcal{C}</math> has cardinality <math>2^{\aleph_{0}}>\aleph_{0}</math>, and since <math>\mathcal{C}\subset\mathbb{R}</math>, there is an injection <math>f:\mathcal{C}\rightarrow\mathbb{R}</math> and thus <math>|\mathbb{R}|\geq 2^{\aleph_{0}}>\aleph_{0}</math>, so <math>\mathbb{R}</math> is uncountable. In fact, it turns out that <math>|\mathbb{R}|=2^{\aleph_{0}}</math>. | |
==See Also== | ==See Also== | ||
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* [[Infinite]] | * [[Infinite]] | ||
* [[Finite]] | * [[Finite]] | ||
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+ | {{stub}} | ||
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+ | [[Category:Definition]] | ||
+ | [[Category:Set theory]] |
Latest revision as of 20:53, 13 October 2019
A set is said to be uncountable if there is no injection
. Assuming the Axiom of choice, every set that is not uncountable is either finite or countably infinite. The most common example of an uncountable set is the set of real numbers
.
Proof that
is uncountable
We give an indirect proof here. This is one of the most famous indirect proofs and was first given by Georg Cantor.
Suppose that the set is countable. Let
be any enumeration of the elements of
(in other words, take an injection
, and denote
).
Consider the decimal expansion of each , say
for all
. Now construct a real number
, by choosing the digit
so that it differs from
by at least 3 and so that
is never equal to 9 or 0. It follows that
differs from
by at least
, so
for every
. Thus,
. However,
is clearly a real number between 0 and 1, a contradiction. Thus our assumption that
is countable must be false, and since
we have that
is uncountable.
An alternative proof uses Cantor's Theorem, which says that for all sets , we have
, where
is the power set of
. First, we note that the Cantor set
has cardinality
, and since
, there is an injection
and thus
, so
is uncountable. In fact, it turns out that
.
See Also
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