Stephens' Constant
Let 
and 
be nonzero integers such that 
(except when 
). Also let 
be the set of primes 
for which 
for some nonnegative integer 
. Then assuming the generalized Riemann hypothesis, Stephens (1976) showed that the density of 
relative to the primes is a rational multiple of
(OEIS A065478), where 
is the 
th prime (Finch 2003).
REFERENCES:
Finch, S. R. "Artin's Constant." §2.4 in Mathematical Constants. Cambridge, England: Cambridge University Press, pp. 104-110, 2003.
Moree, P. "Approximation of Singular Series and Automata." Submitted to Manuscripta Math. 101, 385-399, 2000.
Moree, P. and Stevenhagen, P. "A Two-Variable Artin Conjecture." J. Number Th. 85, 291-304, 2000.
Niklasch, G. "Some Number-Theoretical Constants." https://www.gn-50uma.de/alula/essays/Moree/Moree.en.shtml.
Sloane, N. J. A. Sequence A065478 in "The On-Line Encyclopedia of Integer Sequences."
Stephens, P. J. "Prime Divisor of Second-Order Linear Recurrences, I." J. Number Th. 8, 313-332, 1976.
