Continuum hypothesis

In mathematics, the continuum hypothesis is the assumption that there are as many real numbers as there are elements in the smallest set which is larger than the set of natural numbers.

This statement was first made by Georg Cantor when he studied subsets of the real line. Cantor who introduced sets and cardinal numbers, believed this to be true, but tried in vain to prove it.

From then it stayed, for a long time, a prominent open mathematical problem to resolve. In 1900, David Hilbert included the continuum hypothesis as the first problem, therefore also called "first Hilbert problem", in his famous lecture on 23 problems for the twentieth century.

The first step towards a solution was done in 1938 by Kurt Gödel who showed that - in set theory including the axiom of choice - the (generalized) continuum hypothesis cannot be proved to be false (and thus is consistent with it). Only much later, in 1963, J.Paul Cohen showed that it cannot be proved, either. Hence the continuum hypothesis is independent of the usual (ZFC) axioms of set theory. It therefore constitutes an important, not artificially constructed, example for Gödel's Second Incompleteness Theorem.

Consequently, either the continuum hypothesis or, alternatively, some contradicting assumption could be added to axioms of set theory. But since - in contrast to the situation with the axiom of choice - there is no heuristically convincing reason to choose one of these possibilities, the "working" mathematician usually makes no use of the continuum hypotheses, and if a result depends on it, then it is explicitly mentioned.

Of course, in axiomatic set theory, and especially in the theory of cardinal and ordinal numbers, the situation is different and the consequences of the various choices concerning the continuum hypothesis are extensively studied.

The generalized continuum hypothesis is a much stronger statement involving the initial sequence of transfinite cardinal numbers, and is also independent of ZFC.
In terms of the arithmetic of cardinal numbers (as introduced by Cantor) the continuum hypothesis reads

$\aleph _{0}<2^{\aleph _{0}}$

while the generalized continuum hypothesis is

$\aleph _{n}<2^{\aleph _{n}}(\forall n\in \mathbb {N} )$

David Hilbert 1900

1. _Cantors_ Problem von der Mächtigkeit des Continuums.

Zwei Systeme, d. h. zwei Mengen von gewöhnlichen reellen Zahlen (oder Punkten) heißen nach .Cantor. aequivalent oder von gleicher Mächtigkeit, wenn sie zu einander in eine derartige Beziehung gebracht werden können, daß einer jeden Zahl der einen Menge eine und nur eine bestimmte Zahl der anderen Menge entspricht. Die Untersuchungen von .Cantor. über solche Punktmengen machen einen Satz sehr wahrscheinlich, dessen Beweis jedoch trotz eifrigster Bemühungen bisher noch Niemanden gelungen ist; dieser Satz lautet:

Jedes System von unendlich vielen reellen Zahlen d. h. jede unendliche Zahlen- (oder Punkt)menge ist entweder der Menge der ganzen natürlichen Zahlen 1, 2, 3, ... oder der Menge sämmtlicher reellen Zahlen und mithin dem Continuum, d. h. etwa den Punkten einer Strecke aequivalent; im Sinne der Aeqivalenz giebt es hiernach nur zwei Zahlenmengen, die abzählbare Menge und das Continuum.

Aus diesem Satz würde zugleich folgen, daß das Continuum die nächste Mächtigkeit über die Mächtigkeit der abzählbaren Mengen hinaus bildet; der Beweis dieses Satzes würde mithin eine neue Brücke schlagen zwischen der abzählbaren Menge und dem Continuum.

1. Cantor's problem of the cardinal number of the continuum

Two systems, i. e., two assemblages of ordinary real numbers or points, are said to be (according to Cantor) equivalent or of equal cardinal number, if they can be brought into a relation to one another such that to every number of the one assemblage corresponds one and only one definite number of the other. The investigations of Cantor on such assemblages of points suggest a very plausible theorem, which nevertheless, in spite of the most strenuous efforts, no one has succeeded in proving. This is the theorem:

Every system of infinitely many real numbers, i. e., every assemblage of numbers (or points), is either equivalent to the assemblage of natural integers, 1, 2, 3,... or to the assemblage of all real numbers and therefore to the continuum, that is, to the points of a line; as regards equivalence there are, therefore, only two assemblages of numbers, the countable assemblage and the continuum.

From this theorem it would follow at once that the continuum has the next cardinal number beyond that of the countable assemblage; the proof of this theorem would, therefore, form a new bridge between the countable assemblage and the continuum.