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A065966
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Numbers k such that phi(k) / 2 is prime.
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10
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5, 7, 8, 9, 10, 11, 12, 14, 18, 22, 23, 46, 47, 59, 83, 94, 107, 118, 166, 167, 179, 214, 227, 263, 334, 347, 358, 359, 383, 454, 467, 479, 503, 526, 563, 587, 694, 718, 719, 766, 839, 863, 887, 934, 958, 983, 1006, 1019, 1126, 1174, 1187, 1283, 1307, 1319
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OFFSET
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1,1
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COMMENTS
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This is probably an infinite sequence, but a proof would be nice. Are there infinitely many consecutive terms of the sequence which are also consecutive integers? (For example, 7, 8 and 46, 47.)
Apart from 8, 9, 12 and 18, all the terms of the sequence are safe primes or twice safe primes. It is not known if there are infinitely many safe primes (cf. A005385, A005384). For consecutive terms of the sequence which are also consecutive integers see A066179. - Vladeta Jovovic, Dec 16 2001
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LINKS
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FORMULA
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Numbers k such that A068212(k) = 2.
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EXAMPLE
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phi(46)/2 = 22/2 = 11, a prime.
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MATHEMATICA
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Select[Range[1400], PrimeQ[EulerPhi[#]/2]&] (* Harvey P. Dale, Feb 11 2020 *)
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PROG
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(PARI) for(n=3, 5000, if(isprime(eulerphi(n)/2), print1(n, ", ")))
(PARI) { n=0; for (m=3, 10^9, if (isprime(eulerphi(m)/2), write("b065966.txt", n++, " ", m); if (n==1000, return)) ) } \\ Harry J. Smith, Nov 05 2009
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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