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 A005384 Sophie Germain primes p: 2p+1 is also prime. (Formerly M0731) 380
 2, 3, 5, 11, 23, 29, 41, 53, 83, 89, 113, 131, 173, 179, 191, 233, 239, 251, 281, 293, 359, 419, 431, 443, 491, 509, 593, 641, 653, 659, 683, 719, 743, 761, 809, 911, 953, 1013, 1019, 1031, 1049, 1103, 1223, 1229, 1289, 1409, 1439, 1451, 1481, 1499, 1511, 1559 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Then 2p+1 is called a safe prime: see A005385. Primes p such that the equation phi(x) = 2p has solutions, where phi is the totient function. See A087634 for another such collection of primes. - T. D. Noe, Oct 24 2003 Subsequence of A117360. - Reinhard Zumkeller, Mar 10 2006 Let q = 2n+1. For these n (and q), the difference of two cyclotomic polynomials can be written as a cyclotomic polynomial in x^2: Phi(q,x) - Phi(2q,x) = 2x Phi(n,x^2). - T. D. Noe, Jan 04 2008 A156660(a(n)) = 1; A156874 gives numbers of Sophie Germain primes <= n. - Reinhard Zumkeller, Feb 18 2009 a(n) mod 10 <> 7. - Reinhard Zumkeller, Feb 12 2009 A Sophie Germain prime p is 2, 3 or of the form 6k-1, k >= 1, i.e., p = 5 (mod 6). A prime p of the form 6k+1, k >= 1, i.e., p = 1 (mod 6), cannot be a Sophie Germain prime since 2p+1 is divisible by 3. - Daniel Forgues, Jul 31 2009 Also solutions to the equation: floor(4/A000005(2*n^2+n)) = 1. - Enrique Pérez Herrero, May 03 2012 Solutions of the equation n'+(2n+1)'=2, where n' is the arithmetic derivative of n. - Paolo P. Lava, Aug 10 2012 tau(4*a(n) + 2) = tau(4*a(n)) - 2, for n > 1. - Arkadiusz Wesolowski, Aug 25 2012 eulerphi(4*a(n) + 2) = eulerphi(4*a(n)) + 2, for n > 1. - Arkadiusz Wesolowski, Aug 26 2012 In the spirit of the conjecture related to A217788, we conjecture that for any integers n >= m > 0 there are infinitely many integers b > a(n) such that the number Sum_{k=m..n} a(k)*b^(n-k) is prime. - Zhi-Wei Sun, Mar 26 2013 If k is the product of a Sophie Germain prime p and its corresponding safe prime 2p+1, then a(n) = (k-phi(k))/3, where phi is Euler's totient function. - Wesley Ivan Hurt, Oct 03 2013 Giovanni Resta found the first Sophie German prime which is also Brazilian number (A125134), 28792661 = 1 + 73 + 73^2 + 73^3 + 73^4 = (11111)_73. - Bernard Schott, Mar 07 2019 REFERENCES M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards Applied Math. Series 55, 1964 (and various reprintings), p. 870. A. Peretti, The quantity of Sophie Germain primes less than x, Bull. Number Theory Related Topics, 11:1-3 (1987) 81-92 J. Roberts, Lure of the Integers, Math. Assoc. America, 1992, p. 83. N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS J. S. Cheema, Table of n, a(n) for n = 1..100000. [This replaces an earlier b-file computed by T. D. Noe] M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards, Applied Math. Series 55, Tenth Printing, 1972 [alternative scanned copy]. R. P. Boas & N. J. A. Sloane, Correspondence, 1974 P. Bruillard, S.-H. Ng, E. Rowell, Z. Wang, On modular categories, arXiv preprint arXiv:1310.7050 [math.QA], 2013. C. K. Caldwell, The Prime Glossary, Sophie Germain Prime Del Centina, Andrea. Letters of Sophie Germain preserved in Florence Historia Mathematica, Vol. 32 (2005), 60-75. H. Dubner, Large Sophie Germain Primes, Math. Comp., 65 (1996), 393-396. L. H. Gallardo, O. Rahavandrainy, There are finitely many even perfect polynomials over F_p with p+1 irreducible divisors, Acta Mathematica Universitatis Comenianae, Vol. 83, No. 2, 2016, 261-275. Reikku Kulon, Sublinear arbitrary precision generator of Sophie Germain and safe primes in C (public domain) Victor Meally, Letter to N. J. A. Sloane, no date. Romeo Meštrović, Generalizations of Carmichael numbers I, arXiv:1305.1867v1 [math.NT], May 4, 2013. F. Oort, Prime numbers, 2013. Larry Riddle, Sophie Germain and Fermat's Last Theorem, Agnes Scott College, Math. Dept., Jul, 1999. Maxie D. Schmidt, New Congruences and Finite Difference Equations for Generalized Factorial Functions, arXiv:1701.04741 [math.CO], 2017. Rosemary Sullivan and Neil Watling, Independent divisibility pairs on the set of integers from 1 to n, INTEGERS 13 (2013) #A65. A. Takashi, On Sophie Germain primes, Number theory (Liptovský Ján, 1999), Tatra Mt. Math. Publ. 20 (2000), 65-73. T. Tao, Obstructions to uniformity and arithmetic patterns in the primes, arXiv:math/0505402 [math.NT], 2005. Vmoraru, PlanetMath.org, Germain prime Eric Weisstein's World of Mathematics, Sophie Germain Prime Eric Weisstein's World of Mathematics, Integer Sequence Primes Wikipedia, Sophie Germain prime S. Yates, Sophie Germain primes, in "The mathematical heritage of C. F. Gauss," World Sci. Publ., River Edge, NJ, 1991, pp. 882-886. FORMULA A005097 INTERSECT A000040. - R. J. Mathar, Mar 23 2017 MAPLE A:={}: for n from 1 to 246 do if isprime(2*ithprime(n)+1)=true then A:=A union {ithprime(n)} else A:=A fi od: A:=A; # Emeric Deutsch, Dec 09 2004 MATHEMATICA Select[Prime[Range], PrimeQ[2#+1]&] PROG (MAGMA) [ p: p in PrimesUpTo(1560) | IsPrime(2*p+1) ]; // Klaus Brockhaus, Jan 01 2009 (PARI) select(p->isprime(2*p+1), primes(1000)) \\ In old PARI versions <= 2.4.2, use select(primes(1000), p->isprime(2*p+1)). (PARI) forprime(n=2, 10^3, if(ispseudoprime(2*n+1), print1(n, ", "))) \\ Felix Fröhlich, Jun 15 2014 (PARI) is_A005384=(p->isprime(2*p+1)&&isprime(p));   {A005384_vec(N=100, p=1)=vector(N, i, until(isprime(2*p+1), p=nextprime(p+1)); p)} \\ M. F. Hasler, Mar 03 2020 (GAP) Filtered([1..1600], p->IsPrime(p) and IsPrime(2*p+1)); # Muniru A Asiru, Mar 06 2019 CROSSREFS Cf. A005385, A057331, A005602, A087634. Cf. also A000355, A156541, A156542, A156592, A161896, A156660, A156874, A092816, A023212, A007528 (primes of the form 6k-1). For primes p that remains prime through k iterations of the function f(x) = 2x + 1: this sequence (k=1), A007700 (k=2), A023272 (k=3), A023302 (k=4), A023330 (k=5), A278932 (k=6), A138025 (k=7), A138030 (k=8). Sequence in context: A188534 A214196 A246857 * A118571 A118504 A215714 Adjacent sequences:  A005381 A005382 A005383 * A005385 A005386 A005387 KEYWORD nonn,nice AUTHOR STATUS approved

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Last modified September 18 20:19 EDT 2020. Contains 337173 sequences. (Running on oeis4.)