Larger sieve
# Larger sieve

In number theory, the larger sieve is a sieve invented by Patrick X. Gallagher. The name denotes a heightening of the large sieve. Combinatorial sieves like the Selberg sieve are strongest, when only a few residue classes are removed, while the term large sieve means that this sieve can take advantage of the removal of a large number of up to half of all residue classes. The larger sieve can exploit the deletion of an arbitrary number of classes.

## Statement

Suppose that ${\mathcal {S}}$ is a set of prime powers, N an integer, ${\mathcal {A}}$ a set of integers in the interval [1, N], such that for $q\in {\mathcal {S}}$ there are at most $g(q)$ residue classes modulo $q$, which contain elements of ${\mathcal {A}}$.

Then we have

    $|{\mathcal {A}}|\leq {\frac {\sum _{q\in {\mathcal {S}}}\Lambda (q)-\log N}{\sum _{q\in {\mathcal {S}}}{\frac {\Lambda (q)}{g(q)}}-\log N}},$

provided the denominator on the right is positive.

## Applications

A typical application is the following result, for which the large sieve fails (specifically for $\theta >{\frac {1}{2}}$), due to Gallagher:

If the number of excluded residue classes modulo $p$ varies with $p$, then the larger sieve is often combined with the large sieve. The larger sieve is applied with the set ${\mathcal {S}}$ above defined to be the set of primes for which many residue classes are removed, while the large sieve is used to obtain information using the primes outside ${\mathcal {S}}$.

