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 A156660 Characteristic function of Sophie Germain primes. 20
 0, 0, 1, 1, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 LINKS R. Zumkeller, Table of n, a(n) for n = 0..10000 Wikipedia, Sophie Germain prime FORMULA a(n) = if n and also 2*n+1 is prime then 1 else 0. a(A005384(n)) = 1; a(A138887(n)) = 0; a(A053176(n)) = 0. A156874(n) = Sum_{k=1..n} a(k). - Reinhard Zumkeller, Feb 18 2009 a(n) = A010051(n)*A010051(2*n+1). For n>1 a(n) = floor((floor(phi(n)/(n-1)) + floor(phi(2*n+1)/(2*n)))/2). - Enrique Pérez Herrero, Apr 28 2012 For n>1 a(n) = floor(phi(2*n^2+n)/(2*n^2-2*n)). - Enrique Pérez Herrero, May 02 2012 PROG (Haskell) a156660 n = fromEnum \$ a010051 n == 1 && a010051 (2 * n + 1) == 1 -- _Reinhard Zmkeller_, May 01 2012 (PARI) a(n)=isprime(n)&&isprime(2*n+1) \\ Felix Fröhlich, Aug 11 2014 CROSSREFS Cf. A156659. Cf. A005384, A156874, A092816. Sequence in context: A327174 A269723 A284487 * A155899 A284932 A117814 Adjacent sequences:  A156657 A156658 A156659 * A156661 A156662 A156663 KEYWORD nonn AUTHOR Reinhard Zumkeller, Feb 13 2009 EXTENSIONS Definition corrected by Daniel Forgues, Aug 04 2009 STATUS approved

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Last modified April 10 16:16 EDT 2021. Contains 342845 sequences. (Running on oeis4.)