# Almost prime

Almost prime

In number theory, a natural number is called "k"-almost prime if and only if it has exactly "k" prime factors, counted with multiplicity. More formally, a number "n" is "k"-almost prime if and only if Ω("n") = "k", where Ω("n") is the total number of primes in the prime factorization of "n":

:$Omega\left(n\right) := sum a_i qquadmbox\left\{if\right\}qquad n = prod p_i^\left\{a_i\right\}.$

A natural number is thus prime if and only if it is 1-almost prime, and semiprime if and only if it is 2-almost prime. The set of "k"-almost primes is usually denoted by "P""k". The smallest "k"-almost prime is 2"k". The first few "k"-almost primes are:

:

The number &pi;"k"("n") of positive integers less than or equal to "n" with at most "k" prime divisors (not necessarily distinct) is asymptotic tocite book
last = Tenenbaum
first = Gerald
coauthors =
title = Introduction to Analytic and Probabilistic Number Theory
publisher = Cambridge University Press
date = 1995
isbn = 0521412617

: $pi_k\left(n\right) sim left\left( frac\left\{n\right\}\left\{log n\right\} ight\right) frac\left\{\left(loglog n\right)^\left\{k-1\left\{\left(k - 1\right)!\right\},$a result of Landau. See also the Hardy–Ramanujan theorem.

References

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