Sarnak's constant is the constant
 |  |  |
(1)
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(2)
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(OEIS A065476), where the product is over the odd primes.
Sarnak's Constant
See also
Artin's Constant, Barban's Constant, Feller-Tornier Constant, Heath-Brown-Moroz Constant, Murata's Constant, Prime Products, Quadratic Class Number Constant, Taniguchi's Constant, Twin Primes ConstantExplore with Wolfram|Alpha
References
Niklasch, G. "Some Number-Theoretical Constants." http://www.gn-50uma.de/alula/essays/Moree/Moree.en.shtml.Sarnak, P. C. "Class Numbers of Indefinite Binary Quadratic Forms II." J. Number Th. 21, 333-346, 1985.Sloane, N. J. A. Sequence A065476 in "The On-Line Encyclopedia of Integer Sequences."Referenced on Wolfram|Alpha
Sarnak's ConstantCite this as:
Weisstein, Eric W. "Sarnak's Constant." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/SarnaksConstant.html