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 A238739 Numbers n such that 2^n + 3 and 3*2^n + 1 are both prime. 0
 1, 2, 6, 12, 18, 30 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Intersection of A057732 and A002253. - Joerg Arndt, Mar 04 2014 By checking primality of 2^n+3 for values n in A002253, it follows a(7) > 7033641. - Giovanni Resta, Mar 08 2014 Exponents of second Fermat prime pairs. - Juri-Stepan Gerasimov, Mar 08 2014 From Juri-Stepan Gerasimov, Mar 04 2014: (Start) If prime pair {2^n + (2k+1), (2k+1)*2^n + 1} is called a Fermat prime pair, then numbers n such that 2^n + (2k + 1) and (2k + 1)*2^n + 1 are both prime: for k = 0: 0, 1, 2, 4, 8, 16, ...   the exponents first Fermat prime pairs; for k = 1: 1, 2, 6, 12, 18, 30, ... the exponents second Fermat prime pairs; for k = 2: 1, 3, ...                the exponents third Fermat prime pairs; for k = 3: 2, 4, 6, 20, 174, ...    the exponents fourth Fermat prime pairs; for k = 4: 1, 2, 3, 6, 7, ...       the exponents fifth Fermat prime pairs; for k = 5: 1, 3, 5, 7, ...          the exponents sixth Fermat prime pairs; for k = 6: 2, 8, 20, ...            the exponents seventh Fermat prime pairs; for k = 7: 1, 2, 4, 10, 12, ...     the exponents eighth Fermat prime pairs; for k = 8: for k = 9: 6, ...                   the exponents tenth Fermat prime pairs; for k = 10: 1, 4, 5, 7, 16, ...     the exponents eleventh Fermat prime pairs; for k = 11: for k = 12: 2, 4, 6, 10, 20, 22, ...the exponents thirteenth Fermat prime pairs; for k = 13: 2, 4, 16, 40, 44, ...   the exponents fourteenth Fermat prime pairs; for k = 14: 1, 3, 5, 27, 43, ...    the exponents fifteenth Fermat prime pairs. Semiprimes of the form (2^m+2k+1)*((2k+1)*2^m+1): 4, 9, 25, 35, 77, 91, 209, 289, 319, 481, 527, 533, 901, 989, ... (End) LINKS EXAMPLE a(1) = 1 because 2^1 + 3 = 5 and 3*2^1 + 1 = 7 are both prime, a(2) = 2 because 2^2 + 3 = 7 and 3^2^2 + 1 = 13 are both prime, a(3) = 6 because 2^6 + 3 = 67 and 3*2^6 + 1 = 193 are both prime. MATHEMATICA Select[Range[30], AllTrue[{2^#+3, 3*2^#+1}, PrimeQ]&] (* The program uses the AllTrue function from Mathematica version 10 *) (* Harvey P. Dale, Aug 08 2015 *) PROG (PARI) isok(n) = isprime(2^n + 3) && isprime(3*2^n + 1); \\ Michel Marcus, Mar 04 2014 (MAGMA) [n: n in [0..30] | IsPrime(2^n+3) and IsPrime(3*2^n+1)]; // Arkadiusz Wesolowski, Jan 23 2016 CROSSREFS Cf. A002253, A019434, A039687, A057732, A057733, A238554, A239038, A267984. Sequence in context: A193764 A278484 A159793 * A006511 A113274 A181660 Adjacent sequences:  A238736 A238737 A238738 * A238740 A238741 A238742 KEYWORD nonn,hard,more AUTHOR Juri-Stepan Gerasimov, Mar 04 2014 STATUS approved

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Last modified February 20 00:28 EST 2019. Contains 320329 sequences. (Running on oeis4.)