Georg Ferdinand Ludwig Philipp Cantor ( , ; – January 6, 1918) was a German mathematician
. He created set theory
, which has become a fundamental theory
in mathematics. Cantor established the importance of one-to-one correspondence
between the members of two sets, defined infinite
and well-ordered sets
, and proved that the real number
s are more numerous than the natural number
s. In fact, Cantor's method of proof of this theorem implies the existence of an infinity
of infinities. He defined the cardinal
numbers and their arithmetic. Cantor's work is of great philosophical interest, a fact he was well aware of.
Cantor's theory of transfinite number
s was originally regarded as so counter-intuitive – even shocking – that it encountered resistance
from mathematical contemporaries such as Leopold Kronecker
and Henri Poincaré
and later from Hermann Weyl
and L. E. J. Brouwer
, while Ludwig Wittgenstein
raised philosophical objections
. Cantor, a devout Lutheran
, believed the theory had been communicated to him by God.
[Dauben 2004, pp. 8, 11, 12–13.]
Some Christian theologians
) saw Cantor's work as a challenge to the uniqueness of the absolute infinity in the nature of God
[Dauben 1977, p. 86; Dauben 1979, pp. 120, 143.]
– on one occasion equating the theory of transfinite numbers with pantheism
– a proposition that Cantor vigorously rejected.
The objections to Cantor's work were occasionally fierce: Leopold Kronecker
's public opposition and personal attacks included describing Cantor as a "scientific charlatan", a "renegade" and a "corrupter of youth". Kronecker objected to Cantor's proofs that the algebraic numbers are countable, and that the transcendental numbers are uncountable, results now included in a standard mathematics curriculum. Writing decades after Cantor's death, Wittgenstein lamented that mathematics is "ridden through and through with the pernicious idioms of set theory", which he dismissed as "utter nonsense" that is "laughable" and "wrong". Cantor's recurring bouts of depression from 1884 to the end of his life have been blamed on the hostile attitude of many of his contemporaries,
[Dauben 1979, p. 280: "... the tradition made popular by Arthur Moritz Schönflies blamed Kronecker's persistent criticism and Cantor's inability to confirm his continuum hypothesis" for Cantor's recurring bouts of depression.]
though some have explained these episodes as probable manifestations of a bipolar disorder
[Dauben 2004, p. 1. Text includes a 1964 quote from psychiatrist Karl Pollitt, one of Cantor's examining physicians at Halle Nervenklinik, referring to Cantor's mental illness as "cyclic manic-depression".]
The harsh criticism has been matched by later accolades. In 1904, the Royal Society
awarded Cantor its Sylvester Medal
, the highest honor it can confer for work in mathematics.
defended it from its critics by declaring, "No one shall expel us from the paradise that Cantor has created."
Life of Georg Cantor
Youth and studies
Georg Cantor was born in 1845 in the western merchant colony of Saint Petersburg
, Russia, and brought up in the city until he was eleven. Cantor, the oldest of six children, was regarded as an outstanding violinist. His grandfather Franz Böhm
(1788–1846) (the violinist Joseph Böhm
's brother) was a well-known musician and soloist in a Russian imperial orchestra. Cantor's father had been a member of the Saint Petersburg stock exchange
; when he became ill, the family moved to Germany in 1856, first to Wiesbaden
, then to Frankfurt
, seeking milder winters than those of Saint Petersburg. In 1860, Cantor graduated with distinction from the Realschule in Darmstadt
; his exceptional skills in mathematics, trigonometry
in particular, were noted. In August 1862, he then graduated from the "Höhere Gewerbeschule Darmstadt", now the Technische Universität Darmstadt
. In 1862, Cantor entered the Swiss Federal Polytechnic
. After receiving a substantial inheritance upon his father's death in June 1863,
Cantor shifted his studies to the University of Berlin
, attending lectures by Leopold Kronecker
, Karl Weierstrass
and Ernst Kummer
. He spent the summer of 1866 at the University of Göttingen
, then and later a center for mathematical research. Cantor was a good student, and he received his doctorate degree in 1867.
Teacher and researcher
Cantor submitted his dissertation
on number theory at the University of Berlin in 1867. After teaching briefly in a Berlin girls' school, Cantor took up a position at the University of Halle
, where he spent his entire career. He was awarded the requisite habilitation
for his thesis, also on number theory, which he presented in 1869 upon his appointment at Halle University
In 1874, Cantor married Vally Guttmann. They had six children, the last (Rudolph) born in 1886. Cantor was able to support a family despite modest academic pay, thanks to his inheritance from his father. During his honeymoon in the Harz mountains
, Cantor spent much time in mathematical discussions with Richard Dedekind
, whom he had met two years earlier while on Swiss holiday.
Cantor was promoted to extraordinary professor in 1872 and made full professor in 1879.
To attain the latter rank at the age of 34 was a notable accomplishment, but Cantor desired a chair
at a more prestigious university, in particular at Berlin, at that time the leading German university. However, his work encountered too much opposition for that to be possible.
[Dauben 1979, p. 163.]
Kronecker, who headed mathematics at Berlin until his death in 1891, became increasingly uncomfortable with the prospect of having Cantor as a colleague,
[Dauben 1979, p. 34.]
perceiving him as a "corrupter of youth" for teaching his ideas to a younger generation of mathematicians. Worse yet, Kronecker, a well-established figure within the mathematical community and Cantor's former professor, disagreed fundamentally with the thrust of Cantor's work ever since he intentionally delayed the publication of Cantor's first major publication in 1874.
Kronecker, now seen as one of the founders of the constructive viewpoint in mathematics
, disliked much of Cantor's set theory because it asserted the existence of sets satisfying certain properties, without giving specific examples of sets whose members did indeed satisfy those properties. Whenever Cantor applied for a post in Berlin, he was declined, and it usually involved Kronecker,
so Cantor came to believe that Kronecker's stance would make it impossible for him ever to leave Halle.
In 1881, Cantor's Halle colleague Eduard Heine
died, creating a vacant chair. Halle accepted Cantor's suggestion that it be offered to Dedekind, Heinrich M. Weber
and Franz Mertens
, in that order, but each declined the chair after being offered it. Friedrich Wangerin was eventually appointed, but he was never close to Cantor.
In 1882, the mathematical correspondence between Cantor and Dedekind came to an end, apparently as a result of Dedekind's declining the chair at Halle. Cantor also began another important correspondence, with Gösta Mittag-Leffler
in Sweden, and soon began to publish in Mittag-Leffler's journal ''Acta Mathematica''. But in 1885, Mittag-Leffler was concerned about the philosophical nature and new terminology in a paper Cantor had submitted to ''Acta''.
[Dauben 1979, p. 138.]
He asked Cantor to withdraw the paper from ''Acta'' while it was in proof, writing that it was "... about one hundred years too soon." Cantor complied, but then curtailed his relationship and correspondence with Mittag-Leffler, writing to a third party, "Had Mittag-Leffler had his way, I should have to wait until the year 1984, which to me seemed too great a demand! ... But of course I never want to know anything again about ''Acta Mathematica''."
[Dauben 1979, p. 139.]
Cantor suffered his first known bout of depression in May 1884.
Criticism of his work weighed on his mind: every one of the fifty-two letters he wrote to Mittag-Leffler in 1884 mentioned Kronecker. A passage from one of these letters is revealing of the damage to Cantor's self-confidence:
This crisis led him to apply to lecture on philosophy rather than mathematics. He also began an intense study of Elizabethan literature
thinking there might be evidence that Francis Bacon
wrote the plays attributed to William Shakespeare
(see Shakespearean authorship question
); this ultimately resulted in two pamphlets, published in 1896 and 1897.
Cantor recovered soon thereafter, and subsequently made further important contributions, including his diagonal argument
. However, he never again attained the high level of his remarkable papers of 1874–84, even after Kronecker's death on December 29, 1891.
He eventually sought, and achieved, a reconciliation with Kronecker. Nevertheless, the philosophical disagreements and difficulties dividing them persisted.
In 1889, Cantor was instrumental in founding the German Mathematical Society
and chaired its first meeting in Halle in 1891, where he first introduced his diagonal argument; his reputation was strong enough, despite Kronecker's opposition to his work, to ensure he was elected as the first president of this society. Setting aside the animosity Kronecker had displayed towards him, Cantor invited him to address the meeting, but Kronecker was unable to do so because his wife was dying from injuries sustained in a skiing accident at the time. Georg Cantor was also instrumental in the establishment of the first International Congress of Mathematicians
, which was held in Zürich, Switzerland, in 1897.
Later years and death
After Cantor's 1884 hospitalization, there is no record that he was in any sanatorium
again until 1899.
[Dauben 1979, p. 282.]
Soon after that second hospitalization, Cantor's youngest son Rudolph died suddenly on December 16 (Cantor was delivering a lecture on his views on Baconian theory
and William Shakespeare
), and this tragedy drained Cantor of much of his passion for mathematics.
[Dauben 1979, p. 283.]
Cantor was again hospitalized in 1903. One year later, he was outraged and agitated by a paper presented by Julius König
at the Third International Congress of Mathematicians
. The paper attempted to prove that the basic tenets of transfinite set theory were false. Since the paper had been read in front of his daughters and colleagues, Cantor perceived himself as having been publicly humiliated. Although Ernst Zermelo
demonstrated less than a day later that König's proof had failed, Cantor remained shaken, and momentarily questioning God.
[Dauben 1979, p. 248.]
Cantor suffered from chronic depression for the rest of his life, for which he was excused from teaching on several occasions and repeatedly confined in various sanatoria. The events of 1904 preceded a series of hospitalizations at intervals of two or three years. He did not abandon mathematics completely, however, lecturing on the paradoxes of set theory (Burali-Forti paradox
, Cantor's paradox
, and Russell's paradox
) to a meeting of the ''Deutsche Mathematiker-Vereinigung'' in 1903, and attending the International Congress of Mathematicians at Heidelberg
In 1911, Cantor was one of the distinguished foreign scholars invited to attend the 500th anniversary of the founding of the University of St. Andrews
in Scotland. Cantor attended, hoping to meet Bertrand Russell
, whose newly published ''Principia Mathematica
'' repeatedly cited Cantor's work, but this did not come about. The following year, St. Andrews awarded Cantor an honorary doctorate, but illness precluded his receiving the degree in person.
Cantor retired in 1913, living in poverty and suffering from malnourishment
during World War I
[Dauben 1979, p. 284.]
The public celebration of his 70th birthday was canceled because of the war. In June 1917, he entered a sanatorium for the last time and continually wrote to his wife asking to be allowed to go home. Georg Cantor had a fatal heart attack on January 6, 1918, in the sanatorium where he had spent the last year of his life.
Cantor's work between 1874 and 1884 is the origin of set theory
Prior to this work, the concept of a set was a rather elementary one that had been used implicitly since the beginning of mathematics, dating back to the ideas of Aristotle
. No one had realized that set theory had any nontrivial content. Before Cantor, there were only finite sets (which are easy to understand) and "the infinite" (which was considered a topic for philosophical, rather than mathematical, discussion). By proving that there are (infinitely) many possible sizes for infinite sets, Cantor established that set theory was not trivial, and it needed to be studied. Set theory
has come to play the role of a foundational theory
in modern mathematics, in the sense that it interprets propositions about mathematical objects (for example, numbers and functions) from all the traditional areas of mathematics (such as algebra
) in a single theory, and provides a standard set of axioms to prove or disprove them. The basic concepts of set theory are now used throughout mathematics.
In one of his earliest papers, Cantor proved that the set of real number
s is "more numerous" than the set of natural number
s; this showed, for the first time, that there exist infinite sets of different sizes
. He was also the first to appreciate the importance of one-to-one correspondence
s (hereinafter denoted "1-to-1 correspondence") in set theory. He used this concept to define finite
and infinite set
s, subdividing the latter into denumerable
(or countably infinite) sets and nondenumerable set
s (uncountably infinite sets).
Cantor developed important concepts in topology
and their relation to cardinality
. For example, he showed that the Cantor set
, discovered by Henry John Stephen Smith
in 1875, is nowhere dense
, but has the same cardinality as the set of all real numbers, whereas the rationals
are everywhere dense, but countable. He also showed that all countable dense linear orders
without end points are order-isomorphic to the rational numbers
Cantor introduced fundamental constructions in set theory, such as the power set
of a set ''A'', which is the set of all possible subset
s of ''A''. He later proved that the size of the power set of ''A'' is strictly larger than the size of ''A'', even when ''A'' is an infinite set; this result soon became known as Cantor's theorem
. Cantor developed an entire theory and arithmetic of infinite sets
, called cardinals
, which extended the arithmetic of the natural numbers. His notation for the cardinal numbers was the Hebrew letter
) with a natural number subscript; for the ordinals he employed the Greek letter ω (omega
). This notation is still in use today.
The ''Continuum hypothesis
'', introduced by Cantor, was presented by David Hilbert
as the first of his twenty-three open problems
in his address at the 1900 International Congress of Mathematicians
in Paris. Cantor's work also attracted favorable notice beyond Hilbert's celebrated encomium.
The US philosopher Charles Sanders Peirce
praised Cantor's set theory and, following public lectures delivered by Cantor at the first International Congress of Mathematicians, held in Zurich in 1897, Adolf Hurwitz
and Jacques Hadamard
also both expressed their admiration. At that Congress, Cantor renewed his friendship and correspondence with Dedekind. From 1905, Cantor corresponded with his British admirer and translator Philip Jourdain
on the history of set theory
and on Cantor's religious ideas. This was later published, as were several of his expository works.
Number theory, trigonometric series and ordinals
Cantor's first ten papers were on number theory
, his thesis topic. At the suggestion of Eduard Heine
, the Professor at Halle, Cantor turned to analysis
. Heine proposed that Cantor solve an open problem
that had eluded Peter Gustav Lejeune Dirichlet
, Rudolf Lipschitz
, Bernhard Riemann
, and Heine himself: the uniqueness of the representation of a function
by trigonometric series
. Cantor solved this problem in 1869. It was while working on this problem that he discovered transfinite ordinals, which occurred as indices ''n'' in the ''n''th derived set
of a set ''S'' of zeros of a trigonometric series. Given a trigonometric series f(x) with ''S'' as its set of zeros, Cantor had discovered a procedure that produced another trigonometric series that had ''S''1
as its set of zeros, where ''S''1
is the set of limit point
s of ''S''. If ''S''''k+1''
is the set of limit points of ''S''''k''
, then he could construct a trigonometric series whose zeros are ''S''''k+1''
. Because the sets ''S''''k''
were closed, they contained their limit points, and the intersection of the infinite decreasing sequence of sets ''S'', ''S''1
,... formed a limit set, which we would now call ''S''''ω''
, and then he noticed that ''S''ω
would also have to have a set of limit points ''S''ω+1
, and so on. He had examples that went on forever, and so here was a naturally occurring infinite sequence of infinite numbers ''ω'', ''ω'' + 1, ''ω'' + 2, ...
Between 1870 and 1872, Cantor published more papers on trigonometric series, and also a paper defining irrational number
s as convergent sequences
of rational number
s. Dedekind, whom Cantor befriended in 1872, cited this paper later that year, in the paper where he first set out his celebrated definition of real numbers by Dedekind cut
s. While extending the notion of number by means of his revolutionary concept of infinite cardinality, Cantor was paradoxically opposed to theories of infinitesimal
s of his contemporaries Otto Stolz
and Paul du Bois-Reymond
, describing them as both "an abomination" and "a cholera bacillus of mathematics".
Cantor also published an erroneous "proof" of the inconsistency of infinitesimal
The beginning of set theory as a branch of mathematics is often marked by the publication of Cantor's 1874 paper