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Search: id:A075133
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| A075133 |
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Indices of double-safe primes: p=prime(n) is double-safe: q=(p-1)/2 & r=(q-1)/2 are both prime (and q is safe). |
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+0
2
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| 5, 9, 15, 39, 72, 128, 228, 309, 417, 562, 567, 657, 740, 762, 779, 789, 927, 959, 967, 981, 1214, 1315, 1364, 1632, 1650, 1667, 1785, 1825, 2295, 2425, 2442, 2607, 2709, 2721, 2840, 3085, 3114, 3194, 3401, 3812, 3911, 4428, 4472, 4479, 4645, 4753, 4780
(list; graph; listen)
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OFFSET
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1,1
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COMMENT
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prime p is safe if q=(p-1)/2 is prime, so p is double safe if also r=(q-1)/2 is prime. Safe primes are in A005385, indices of triple-safe primes are in A075314
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EXAMPLE
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72 is a member because p(72)=359, q=(p-1)/2=179 and r=(q-1)2=89 are primes.
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MATHEMATICA
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se2=Select[(Select[(Prime[Range[20000]]-1)/2, PrimeQ]-1)/2, PrimeQ]; Map[PrimePi, Map[2(2*#+1)+1&, se2]]
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CROSSREFS
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Cf. A005385, A075314.
Sequence in context: A074953 A023498 A062516 this_sequence A066081 A076856 A099541
Adjacent sequences: A075130 A075131 A075132 this_sequence A075134 A075135 A075136
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KEYWORD
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nonn
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AUTHOR
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Zak Seidov (zakseidov(AT)yahoo.com), Sep 04 2002
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