Sieve theory is a set of general techniques in number theory, designed to count, or more realistically to estimate the size of, sifted sets of integers. The prototypical example of a sifted set is the set of prime numbers up to some prescribed limit X. Correspondingly, the prototypical example of a sieve is the sieve of Eratosthenes, or the more general Legendre sieve. The direct attack on prime numbers using these methods soon reaches apparently insuperable obstacles, in the way of the accumulation of error terms. In one of the major strands of number theory in the twentieth century, ways were found of avoiding some of the difficulties of a frontal attack with a naive idea of what sieving should be.
One successful approach is to approximate a specific sifted set of numbers (e.g. the set of prime numbers) by another, simpler set (e.g. the set of almost prime numbers), which is typically somewhat larger than the original set, and easier to analyze. More sophisticated sieves also do not work directly with sets per se, but instead count them according to carefully chosen weight functions on these sets (options for giving some elements of these sets more "weight" than others). Furthermore, in some modern applications, sieves are used not to estimate the size of a sifted set, but to produce a function that is large on the set and mostly small outside it, while being easier to analyze than the characteristic function of the set.
We start with some finite sequence of non-negative numbers . In the most basic case this sequence is just the indicator function of some set we want to sieve. However this abstraction allows for more general situations. Next we introduce a general set of prime numbers called the sifting range and their product up to as a function .
The goal of sieve theory is to estimate the sifting function
In the case of this just counts the cardinality of a subset of of numbers, that are coprime to the prime factors of .
We can rewrite this as
by using the Möbius function and some functions induced by the elements of
One assumes then that can be written as
where is a density, meaning a multiplicative function such that
and is an approximation of and is some remainder term. The sifting function becomes
or in short
One tries then to estimate the sifting function by finding upper and lower bounds for respectively and .
The partial sum of the sifting function alternately over- and undercounts, so the remainder term will be huge. Brun's idea to improve this was to replace in the sifting function with a weight sequence consisting of restricted Möbius functions. Choosing two appropriate sequences and and denoting the sifting functions with and , one can get lower and upper bounds for the original sifting functions
Notation: a word of caution regarding the notation, in the literature one often identifies the set of sequences with the set itself. This means one writes to define a sequence . Also in the literature the sum is sometimes notated as the cardinality of some set , while we have defined to be already the cardinality of this set. We used to denote the set of primes.
Modern sieves include the Brun sieve, the Selberg sieve, the Turán sieve, the large sieve, and the larger sieve. One of the original purposes of sieve theory was to try to prove conjectures in number theory such as the twin prime conjecture. While the original broad aims of sieve theory still are largely unachieved, there have been some partial successes, especially in combination with other number theoretic tools. Highlights include:
The techniques of sieve theory can be quite powerful, but they seem to be limited by an obstacle known as the parity problem, which roughly speaking asserts that sieve theory methods have extreme difficulty distinguishing between numbers with an odd number of prime factors and numbers with an even number of prime factors. This parity problem is still not very well understood.
Compared with other methods in number theory, sieve theory is comparatively elementary, in the sense that it does not necessarily require sophisticated concepts from either algebraic number theory or analytic number theory. Nevertheless, the more advanced sieves can still get very intricate and delicate (especially when combined with other deep techniques in number theory), and entire textbooks have been devoted to this single subfield of number theory; a classic reference is (Halberstam & Richert 1974) and a more modern text is (Iwaniec & Friedlander 2010).
The sieve methods discussed in this article are not closely related to the integer factorization sieve methods such as the quadratic sieve and the general number field sieve. Those factorization methods use the idea of the sieve of Eratosthenes to determine efficiently which members of a list of numbers can be completely factored into small primes.