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In mathematics, the continuum hypothesis is a hypothesis about the possible sizes of infinite sets. Georg
Cantor introduced the concept of cardinality to compare the sizes
of infinite sets, and he showed that the set of integers is strictly smaller than the
set of real numbers. The continuum hypothesis states the following:
There is no set whose size is strictly between that of the integers and that of the real numbers.
Or mathematically speaking, noting that the cardinality for the integers
is ("aleph-null") and the cardinality for the real numbers is , the continuum hypothesis says:
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The real numbers have also been called the continuum, hence the name. There is also a generalization of the continuum
hypothesis called the generalized continuum hypothesis, which is described at the end of this article.
The size of a set
To state the hypothesis formally, we need a definition: we say that two sets S and T have the same
cardinality or cardinal number if there exists a
bijection . Intuitively, this means that it is possible to "pair off" elements of S with elements of
T in such a fashion that every element of S is paired off with exactly one element of T and vice
versa. Hence, the set {banana, apple, pear} has the same cardinality of {yellow, red, green}.
With infinite sets such as the set of integers or rational numbers, things are more complicated to show. Consider the set of all rational numbers. One
might naively suppose that there are more rational numbers than integers, and fewer rational numbers than real numbers, thus
disproving the continuum hypothesis. However, it turns out that the rational numbers can be placed in one-to-one correspondence
with the integers, and therefore the set of rational numbers is the same size as the set of integers: they are both countable sets. Cantor's diagonal argument shows that the integers and the continuum do not have the same
cardinality.
The continuum hypothesis states that every subset of the continuum (= the real numbers) which contains the integers either has the same cardinality as the
integers or the same cardinality as the continuum.
Investigating the continuum hypothesis
If a set S was found that disproved the continuum hypothesis, it would be impossible to make a one-to-one
correspondence between S and the set of integers, because there would always be elements of set S that were
"left over". Similarly, it would be impossible to make a one-to-one correspondence between S and the set of real
numbers, because there would always be real numbers that were "left over".
Impossibility of proof and disproof
Cantor believed the continuum hypothesis to be true and tried for many years to prove it, in vain. It became the first on
David Hilbert's list of important open questions that was presented at the International Mathematical Congress in the year 1900 in Paris.
Kurt Gödel showed in 1940 that the continuum hypothesis (CH for short)
cannot be disproved from the standard Zermelo-Fraenkel set
theory axiom system, even if the axiom of choice is adopted.
Paul Cohen showed in 1963 that CH cannot be proven from those same axioms
either. Hence, CH is independent of the Zermelo-Fraenkel axiom system and of the axiom of choice. Both of these results
assume that the Zermelo-Fraenkel axioms themselves don't contain a contradiction, something that's widely believed to be
true.
As such it is not surprising that there should be statements which cannot be proven nor disproven within a given axiom system;
in fact the content of Gödel's
incompleteness theorem is that such statements always exist if the axiom system is strong enough and without contradictions.
The independence of CH was still unsettling however, because it was the first concrete example of an important, interesting
question of which it could be proven that it could not be decided either way from the universally accepted basic system of axioms
on which mathematics is built.
The continuum hypothesis is closely related to many statements in analysis, point set topology and measure theory. As a result of its independence, many substantial conjectures in those fields have subsequently been shown to be independent as well.
It is interesting to note that Gödel believed strongly that CH is false. To him, his independence of proof only showed that
the prevalent set of axioms was defective. Gödel was a platonist and therefore
had no problems with asserting truth and falsehood of statements independent of their provability. Cohen, however, was a
formalist, but even he tended towards rejecting CH. Nowadays, most researchers in the field are either neutral or reject CH.
Generally speaking, mathematicians who favour a "rich" and "large" universe of sets are against CH, while those favoring a "neat" and "controllable" universe favor CH.
Chris Freiling in 1986 presented an argument against CH: he showed that
the negation of CH is equivalent to a statement about probabilities which he calls "intuitively true", but others have
disagreed.
The generalized continuum hypothesis
The generalized continuum hypothesis (GCH) states that if an infinite set's cardinality lies between that of an
infinite set S and that of the power set of S, then it either
has the same cardinality as the set S or the same cardinality as the power set of S: there are no in-betweens.
This is a generalization of the continuum hypothesis since the continuum has the same cardinality as the power set of the integers. GCH is also independent of the Zermelo-Fraenkel set theory
axioms and it implies the axiom of choice.
See also
References
- Nancy McGough.: The Continuum
Hypothesis .
- Cohen, P. J.: Set Theory and the Continuum Hypothesis. New York:
W. A. Benjamin, 1966.
- Gödel, K.: The Consistency of the Continuum-Hypothesis.
Princeton, NJ: Princeton University Press, 1940.
- Gödel, K.: What is Cantor's Continuum Problem?, reprinted in Benacerraf and Putnam's collection Philosophy of
Mathematics, 2nd ed., Cambridge University Press, 1983. An outline of Gödel's arguments against CH.
- H. G. Dales and W. H. Woodin: An Introduction to Independence for Analysts. Cambridge (1987).
- Chris Freiling: "Axioms of Symmetry: Throwing Darts at the Real Number Line," Journal of Symbolic Logic, Volume 51
(1986), Issue 1, pp. 190-200.
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