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A295125
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Numbers k such that (19*10^k + 521)/9 is prime.
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0
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1, 2, 4, 7, 34, 218, 253, 310, 362, 370, 568, 574, 607, 919, 1615, 3968, 5950, 10435, 11887, 176932
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OFFSET
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1,2
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COMMENTS
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For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 1 followed by the digits 69 is prime (see Example section).
a(21) > 2*10^5.
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LINKS
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EXAMPLE
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2 is in this sequence because (19*10^2 + 521)/9 = 269 is prime.
Initial terms and associated primes:
a(1) = 1, 79;
a(2) = 2, 269;
a(3) = 4, 21169;
a(4) = 7, 21111169;
a(5) = 34, 21111111111111111111111111111111169; etc.
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MATHEMATICA
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Select[Range[0, 100000], PrimeQ[(19*10^# + 521)/9] &]
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PROG
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(PARI) isok(k) = isprime((19*10^k + 521)/9); \\ Michel Marcus, Nov 15 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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