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A294914
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Numbers k such that (14*10^k - 131)/9 is prime.
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0
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4, 7, 10, 27, 40, 43, 55, 61, 63, 117, 259, 417, 427, 561, 568, 889, 1335, 26625, 36613, 45261, 47205
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OFFSET
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1,1
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COMMENTS
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For k > 1, numbers k such that the digit 6 followed by k-2 occurrences of the digit 5 followed by the digits 47 is prime (see Example section).
a(22) > 2*10^5.
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LINKS
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EXAMPLE
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4 is in this sequence because (14*10^4 - 131)/9 = 15541 is prime.
Initial terms and associated primes:
a(1) = 4, 15541;
a(2) = 7, 15555541;
a(3) = 10, 15555555541;
a(4) = 27, 1555555555555555555555555541;
a(5) = 40, 15555555555555555555555555555555555555541; etc.
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MATHEMATICA
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Select[Range[1, 100000], PrimeQ[(14*10^# - 131)/9] &]
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CROSSREFS
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KEYWORD
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nonn,more,hard
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AUTHOR
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STATUS
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approved
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