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Major progress in prime number theory

Krishnaswami Alladi

The Green-Tao theorem resolves an important special case of the Erdös-Turan conjecture

Kumbakonam: Professor Terence Tao of the University of California, Los Angeles (UCLA), was awarded the 2006 SASTRA's Ramanujan Prize at the International Conference on Number Theory and Combinatorics at the Srinivasa Ramanujan Centre, SASTRA University, Kumbakonam.

This $10,000 prize comes on the heels of the Fields Medal that was awarded to Professor Tao in August for revolutionary contributions to several areas of mathematics.

Following the award ceremony on Ramanujan's birthday at Kumbakonam, Professor Tao delivered the Ramanujan Commemoration Lecture entitled "Long arithmetic progressions of primes," in which he reported major progress in prime number theory based on his recent work with Professor Ben Green of Cambridge University.

One of the most famous unsolved problems in mathematics is the Prime Twins Conjecture, which asserts that there are infinitely many prime pairs that differ by 2. More generally, the prime k-tuples conjecture states that if a k-tuple is admissible, then there are infinitely many such k-tuples of primes. Here by admissible one means that the k-tuple must satisfy certain non-divisibility conditions.

If the prime k-tuples conjecture is true, then it follows that there are arbitrarily long arithmetic progressions of primes. For example, 7, 37, 67, 97, 127, 157, is an arithmetic progression of 6 primes with common difference 30.

Sieve theory was developed in the 20th century to attack problems such as the k-tuples conjecture. Although this conjecture is still unsolved, sieve methods have succeeded in establishing similar results for almost primes, namely, those integers with very few prime factors, but not for the primes themselves.

Thus, the world was astonished when Professor Tao and Professor Green proved in 2003 that there are arbitrarily long arithmetic progressions of primes. The road to the Green-Tao theorem has been long, and in his lecture, Professor Tao surveyed the history of the problem and described the techniques that led to the recent breakthrough.

The first major advance was made in 1939 by van der Corput, who showed that there are infinitely many triples of primes in arithmetic progression. He used the circle method, originally invented by Hardy and Ramanujan to estimate the number of partitions of an integer and subsequently improved by Hardy and Littlewood to apply to a wide class of problems in additive number theory.

van der Corput's result was improved in 1981 by the British mathematician Heath Brown, who showed that there are infinitely many quadruples in arithmetic progression of which three are primes, and the fourth an almost prime with at most two prime factors. That such an improvement came after more than 40 years indicates the difficulty of the problem.

Another problem was the study of finite arithmetic progressions within sets of positive density. This was pioneered by the 1958 Fields medallist K.F. Roth, who in 1956 showed that any set of integers with positive density contains infinitely many triples in arithmetic progression. This study culminated in 1975 with the grand result of the Hungarian mathematician Szemeredi, who proved that any set of integers with positive density contains arithmetic progressions of arbitrary length. Professor Tim Gowers of Cambridge University, who won the Fields Medal in 1994, has recently given a simpler proof of Szemeredi's theorem. It is to be noted that since the primes have zero density, Szemeredi's theorem does not imply that there are arbitrarily long arithmetic progressions of primes.

Professor Green was a Ph.D student of Professor Gowers, who introduced him to Szemeredi's theorem. One of Professor Green's first major accomplishments was the result that any subset of the primes, which has relative positive density, contains infinitely many triples on arithmetic progressions. Professor Tao and Professor Green then corresponded due to their common interest on such problems. They studied the general problem of arithmetic progressions in sparse sets of integers. By combining ideas from ergodic theory, the techniques of Professor Gowers, and repeated use of Szemeredi's theorem, they were able to prove the astonishing result that there are arbitrarily long arithmetic progressions of primes. The ingredients of the proof were put together when Professor Green visited Professor Tao at UCLA in 2003.

The great Hungarian mathematicians Paul Erdös and Paul Turan conjectured that if A is an infinite set of integers the sum of whose reciprocals is divergent, then there are arbitrarily long arithmetic progressions in A. Since the sum of the reciprocals of the primes is a divergent series, the Green-Tao theorem is a special case of the Erdös-Turan conjecture, which remains unsolved in full generality. Erdös has offered $10,000 for a resolution of this conjecture. The Green-Tao theorem resolves an important special case of the Erdös-Turan conjecture and is a phenomenal achievement by two brilliant young mathematicians. Thus, it was a fitting tribute to Ramanujan that this great work was presented in his hometown on his birthday.

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