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A295030
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Numbers k such that 4*10^k + 91 is prime.
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0
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1, 2, 3, 8, 10, 15, 31, 32, 77, 106, 191, 197, 525, 557, 1958, 3848, 12771, 13348, 13507, 23164, 55932, 67375, 68020, 95407, 95672, 109142, 113905, 150577, 185905, 261248
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OFFSET
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1,2
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COMMENTS
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For k > 1, numbers such that the digit 4 followed by k-2 occurrences of the digit 0 followed by the digits 91 is prime (see Example section).
a(31) > 3*10^5.
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LINKS
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EXAMPLE
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2 is in this sequence because 4*10^2 + 91 = 491 is prime.
Initial terms and primes associated:
a(1) = 1, 131;
a(2) = 2, 491;
a(3) = 3, 4091;
a(4) = 8, 400000091;
a(5) = 10, 40000000091; etc.
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MATHEMATICA
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Select[Range[0, 100000], PrimeQ[4*10^# + 91] &]
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PROG
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(PARI) isok(k) = isprime(4*10^k + 91); \\ Michel Marcus, Nov 13 2017
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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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EXTENSIONS
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STATUS
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approved
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