Vojtěch Jarník (; 22 December 1897 – 22 September 1970) was a Czech mathematician. He worked for many years as a professor and administrator at Charles University, and helped found the Czechoslovak Academy of Sciences. He is the namesake of Jarník's algorithm for minimum spanning trees.

Jarník worked in number theory, mathematical analysis, and graph algorithms. He has been called "probably the first Czechoslovak mathematician whose scientific works received wide and lasting international response". and his older brother, Hertvík Jarník, also became a professor of linguistics. Despite this background, Jarník learned no Latin at his gymnasium (the C.K. české vyšší reálné gymnasium, Ječná, Prague), so when he entered Charles University in 1915 he had to do so as an extraordinary student until he could pass a Latin examination three semesters later.

While keeping his position at Charles University, he studied with Edmund Landau at the University of Göttingen from 1923 to 1925 and again from 1927 to 1929. On his first return to Charles University he defended his habilitation, and on his return from the second visit, he was given a chair in mathematics as an extraordinary professor. He was promoted to full professor in 1935 and later served as Dean of Sciences (1947–1948) and Vice-Rector (1950–1953). He retired in 1968.

He died on 22 September 1970, at the age of 72.

Another theorem of Jarník in this area shows that, for any closed convex curve in the plane with a well-defined length, the absolute difference between the area it encloses and the number of integer points it encloses is at most its length.

Jarník also published several results in Diophantine approximation, the study of the approximation of real numbers by rational numbers.

He proved (1928–1929) that the badly approximable real numbers (the ones with bounded terms in their continued fractions) have Hausdorff dimension one. This is the same dimension as the set of all real numbers, intuitively suggesting that the set of badly approximable numbers is large. He also considered the numbers

for which there exist infinitely many good rational approximations , with

:<math>\left| x-\frac{p}{q}\right|<\frac{1}{q^k}</math>

for a given exponent , and proved (1929) that these have the smaller Hausdorff dimension . The second of these results was later rediscovered by Besicovitch. Besicovitch used different methods than Jarník to prove it, and the result has come to be known as the Jarník–Besicovitch theorem.

Mathematical analysis

Jarník's work in real analysis was sparked by finding, in the unpublished works of Bernard Bolzano, a definition of a continuous function that was nowhere differentiable. Bolzano's 1830 discovery predated the 1872 publication of the Weierstrass function, previously considered to be the first example of such a function. Based on his study of Bolzano's function, Jarník was led to a more general theorem: If a real-valued function of a closed interval does not have bounded variation in any subinterval, then there is a dense subset of its domain on which at least one of its Dini derivatives is infinite. This applies in particular to the nowhere-differentiable functions, as they must have unbounded variation in all intervals. Later, after learning of a result by Stefan Banach and Stefan Mazurkiewicz that generic functions (that is, the members of a residual set of functions) are nowhere differentiable, Jarník proved that at almost all points, all four Dini derivatives of such a function are infinite. Much of his later work in this area concerned extensions of these results to approximate derivatives.

Combinatorial optimization

thumb|upright|Animation of [[Jarník's algorithm for minimum spanning trees]]

In computer science and combinatorial optimization, Jarník is known for an algorithm for constructing minimum spanning trees that he published in 1930, in response to the publication of Borůvka's algorithm by another Czech mathematician, Otakar Borůvka. Jarník's algorithm builds a tree from a single starting vertex of a given weighted graph by repeatedly adding the cheapest connection to any other vertex, until all vertices have been connected.

The same algorithm was later rediscovered in the late 1950s by Robert C. Prim and Edsger W. Dijkstra. It is also known as Prim's algorithm or the Prim–Dijkstra algorithm.

He also published a second, related, paper with (1934) on the Euclidean Steiner tree problem. In this problem, one must again form a tree connecting a given set of points, with edge costs given by the Euclidean distance. However, additional points that are not part of the input may be added to make the overall tree shorter. This paper is the first serious treatment of the general Steiner tree problem (although it appears earlier in a letter by Gauss), and it already contains "virtually all general properties of Steiner trees" later attributed to other researchers.

Recognition and legacy

Jarník was a member of the Czech Academy of Sciences and Arts, from 1934 as an extraordinary member and from 1946 as a regular member. as is Jarníkova Street in the Chodov district of Prague. A series of postage stamps published by Czechoslovakia in 1987 to honor the 125th anniversary of the Union of Czechoslovak mathematicians and physicists included one stamp featuring Jarník together with Joseph Petzval and Vincenc Strouhal.

A conference was held in Prague, in March 1998, to honor the centennial of his birth.

Since 2002, ceremonial Jarník's lecture is held every year at Faculty of Mathematics and Physics, Charles University, in a lecture hall named after him.

Selected publications

Jarník published 90 papers in mathematics, including:

  • . A function with unbounded variation in all intervals has a dense set of points where a Dini derivative is infinite.

References

Further reading

  • .