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 A000355 Primes = 3, 9, 11 (mod 20) such that 2p+1 is also prime. 3
 3, 11, 23, 29, 83, 89, 131, 191, 251, 431, 443, 491, 509, 683, 743, 809, 911, 1031, 1049, 1103, 1223, 1229, 1289, 1409, 1451, 1511, 1583, 1811, 1889, 1931, 2003, 2063, 2069, 2129, 2351, 2543, 2549, 2903, 2963, 2969, 3023, 3329, 3389, 3449, 3491, 3623, 3803 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS a(n) = (A000353(n)-1)/2. - Reinhard Zumkeller, Feb 10 2009 REFERENCES R. A. J. Matthews, Maximally periodic reciprocals, Bull. Institute of Mathematics and Its Applications, vol. 28, p. 147-148, 1992. LINKS R. Zumkeller, Table of n, a(n) for n = 1..1000 MATHEMATICA Select[Prime[Range[1000]], MatchQ[Mod[#, 20], 3|9|11] && PrimeQ[2#+1]&] (* Jean-François Alcover, Feb 07 2016 *) PROG (PARI) is(n)=my(k=n%20); (k==3||k==9||k==11) && isprime(2*n+1) && isprime(n) \\ Charles R Greathouse IV, Nov 20 2014 CROSSREFS Subset of A005384. Cf. A000353. Sequence in context: A289765 A141226 A049491 * A163769 A100860 A018630 Adjacent sequences:  A000352 A000353 A000354 * A000356 A000357 A000358 KEYWORD nonn,easy AUTHOR R. A. J. Matthews [ 100265.3005(AT)compuserve.com ] EXTENSIONS More terms from Reinhard Zumkeller, Feb 10 2009 STATUS approved

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Last modified April 10 14:07 EDT 2021. Contains 342845 sequences. (Running on oeis4.)