# Almost-prime number

From Encyclopedia of Mathematics

A natural number of the form

where the are prime numbers and is a constant. Prime numbers are the special case of almost-prime numbers for . There are theorems for almost-prime numbers that generalize theorems on the distribution of prime numbers in the set of natural numbers. Several additive problems that have not yet been solved for prime numbers have been solved for almost-prime numbers.

#### Comments

See also Sieve method.

An example of a result on the distribution of almost-prime numbers generalizing the corresponding one on prime numbers is the following. Let be the number of square-free almost-prime numbers . Then (cf. [a1], Sect. 22.18)

#### References

[a1] | G.H. Hardy, E.M. Wright, "An introduction to the theory of numbers" , Clarendon Press (1965) |

**How to Cite This Entry:**

Almost-prime number. B.M. Bredikhin (originator),

*Encyclopedia of Mathematics.*URL: http://www.encyclopediaofmath.org/index.php?title=Almost-prime_number&oldid=12988

This text originally appeared in Encyclopedia of Mathematics - ISBN 1402006098