Meissel–Lehmer algorithm

The Meissel–Lehmer algorithm (after Ernst Meissel and Derrick Henry Lehmer) is an algorithm that computes the prime-counting function.

Description

Key functions

Let $p_{1},p_{2},\ldots ,p_{n}$ be the first n primes. For natural number a ≥ 1, define

$\varphi (x,a):=\left|\left\{n\leq x:p|n\implies p>p_{a}\right\}\right|,$

which counts natural numbers no greater than x with all prime factors greater $p_{a}$. Also define for natural number k,

$P_{k}(x,a):=\left|\left\{n\leq x:n=q_{1}q_{2}\cdots q_{k},~{\text{with}}~q_{1},\ldots ,q_{k}>p_{a}\right\}\right|,$

which counts natural numbers no greater than x with exactly k prime factors, all larger than $p_{a}$. With these, we have

$\varphi (x,a)=\sum _{k=0}^{\infty }P_{k}(x,a),$

where the sum only has finitely many nonzero terms, because $P_{k}(x,a)=0$ when $p_{a}^{k}>x$. Using the fact that $P_{1}(x,a)=\pi (x)-a$, we get

$\pi (x)=\varphi (x,a)+a-1-\sum _{k=2}^{\infty }P_{k}(x,a),$

which prove that one may compute $\pi (x)$ by computing $\varphi (x,a)$ and $P_{k}(x,a)$ for k ≥ 2. This is what the Meissel–Lehmer algorithm does.

Formula for P_k(x, a)

For k = 2, we get the following formula for $P_{k}(x,a)$:

${\begin{aligned}P_{2}(x,a)&=\left|\left\{n:n\leq x,~n=p_{b}p_{c},~{\text{with}}~a<b\leq c\right\}\right|\\&=\sum _{b=a+1}^{\pi (x^{1/2})}\left|\left\{n:n\leq x,~n=p_{b}p_{c},~{\text{with}}~b\leq c\leq \pi \left({\frac {x}{p_{b}}}\right)\right\}\right|\\&=\sum _{b=a+1}^{\pi (x^{1/2})}\left(\pi \left({\frac {x}{p_{b}}}\right)-(b-1)\right)\\&={\binom {a}{2}}-{\binom {\pi (x^{1/2})}{2}}+\sum _{b=a+1}^{\pi (x^{1/2})}\pi \left({\frac {x}{p_{b}}}\right).\end{aligned}}$

For k ≥ 3, the identities for $P_{k}(x,a)$ can be derived similarly.

Meissel already found that for $k\geq 3,P_{k}(x,a)=0$ if $a=\pi (x^{1/3})$. He used the resulting equation for calculations of $\pi (x)$ for big values of $x$.

Meissel calculated $\pi (x)$ up to $x=1e9$, but he narrowly missed the correct result for the biggest value of x.

Expanding 𝜑(x, a)

Using the recurrence

$\varphi (x,a)=\varphi (x,a-1)-\varphi \left({\frac {x}{p_{a}}},a-1\right),$

$\varphi (x,a)$ may be expanded. Each summand, in turn, may be expanded in the same way.

Combining the terms

The only thing that remains to be done is evaluating $\varphi (x,a)$ and $P_{k}(x,a)$ for k ≥ 2, for certain values of x and a. This can be done by direct sieving and using the above formulas.

Lehmer analyzed especially the calculation of $\varphi (x,a)$ and used already computers to calculate the correct value of $a=\pi (1e9)$.

Modern variants

Jeffrey Lagarias, Victor Miller and Andrew Odlyzko published a realisation of the algorithm which computes $\pi (x)$ in time $O(x^{2/3+\varepsilon })$ and space $O(x^{1/3+\varepsilon })$ for any $\varepsilon >0$. Upon setting $a=\pi (x^{1/3})$, the tree of $\varphi (x,a)$ has $O(x^{2/3})$ leaf nodes.

This extended Meissel-Lehmer algorithm needs less computing time than the algorithm developed by Meissel and Lehmer, especially for big values of x.

Further improvements are given by M. Deleglise and J. Rivat in 1996.