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A257034
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Numbers k such that 7*R_(k+2) + 2*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length n.
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0
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0, 1, 3, 9, 16, 30, 45, 123, 171, 295, 324, 2785, 2791, 3910, 4015, 4050, 6319, 6415, 14670
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OFFSET
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1,3
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COMMENTS
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Also, numbers k such that (718*10^n-7)/9 is prime.
Terms from Kamada.
a(20) > 10^5.
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LINKS
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Table of n, a(n) for n=1..19.
Makoto Kamada, Near-repdigit numbers of the form ABAA...AA.
Makoto Kamada, Prime numbers of the form 7977...77.
Index entries for primes involving repunits.
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EXAMPLE
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For k=3, 7*R_5 + 2*10^3 = 77777 + 2000 = 79777 which is prime.
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MATHEMATICA
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Select[Range[0, 30000], PrimeQ[(718*10^#-7)/9 ] &]
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PROG
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(MAGMA) [n: n in [0..400] | IsPrime((718*10^n-1) div 9)]; // Vincenzo Librandi, Apr 15 2015
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CROSSREFS
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Cf. A002275.
Sequence in context: A244475 A212870 A306447 * A232167 A058538 A197531
Adjacent sequences: A257031 A257032 A257033 * A257035 A257036 A257037
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KEYWORD
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more,hard,nonn
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AUTHOR
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Robert Price, Apr 14 2015
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STATUS
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approved
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