Mills' theorem states that there exists a real constant 
such that 
is prime for all positive integers 
(Mills 1947). While for each value of 
, there are uncountably many possible values of 
such that 
is prime for all positive integers 
(Caldwell and Cheng 2005), it is possible to define Mills'
constant as the least 
such that
 |
is prime for all positive integers 
, giving a value of
 |
(OEIS A051021).
is therefore given by the next
prime after 
,
and the values of 
are known as Mills' primes (Caldwell and Cheng 2005).
Caldwell and Cheng (2005) computed more than 6850 digits of 
assuming the truth of the Riemann
hypothesis. Proof of primality of the 13 Mills prime
in Jul. 2013 means that approximately 
digits are now known.
It is not known if 
is irrational.
