# Mills' constant

In number theory, Mills' constant is defined as the smallest positive real number A such that the floor of the double exponential function

$\lfloor A^{3^{n}} \rfloor$

is a prime number, for all positive integers n. This constant is named after William H. Mills who proved in 1947 the existence of A based on results of Guido Hoheisel and Albert Ingham on the prime gaps. Its value is unknown, but if the Riemann hypothesis is true, it is approximately 1.3063778838630806904686144926... (sequence A051021 in OEIS).

## Mills primes

The primes generated by Mills' constant are known as Mills primes; if the Riemann hypothesis is true, the sequence begins

2, 11, 1361, 2521008887, 16022236204009818131831320183, 4113101149215104800030529537915953170486139623539759933135949994882770404074832568499, ... (sequence A051254 in OEIS).

If ai denotes the ith prime in this sequence, then ai can be calculated as the smallest prime number larger than $a_{i-1}^3$. In order to ensure that rounding $A^{3^n}$, for n = 1, 2, 3, …, produces this sequence of primes, it must be the case that $a_i < (a_{i-1}+1)^3$. The Hoheisel–Ingham results guarantee that there exists a prime between any two sufficiently large cubic numbers, which is sufficient to prove this inequality if we start from a sufficiently large first prime $a_1$. The Riemann hypothesis implies that there exists a prime between any two consecutive cubes, allowing the sufficiently large condition to be removed, and allowing the sequence of Mills' primes to begin at a1 = 2.

Currently, the largest known Mills prime (under the Riemann hypothesis) is

$\displaystyle (((((((((2^3+3)^3+30)^3+6)^3+80)^3+12)^3+450)^3+894)^3+3636)^3+70756)^3+97220,$

which is 20,562 digits long.

## Numerical calculation

By calculating the sequence of Mills primes, one can approximate Mills' constant as

$A\approx a(n)^{1/3^n}.$

Caldwell & Cheng (2005) used this method to compute almost seven thousand base 10 digits of Mills' constant under the assumption that the Riemann hypothesis is true. There is no closed-form formula known for Mills' constant, and it is not even known whether this number is rational (Finch 2003).