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A294942
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Numbers k such that (74*10^k + 313)/9 is prime.
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0
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0, 2, 6, 15, 18, 26, 51, 75, 86, 114, 257, 275, 537, 753, 914, 1944, 18431, 18741, 21281, 28272, 31002, 71621, 84170, 110961
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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 8 followed by k-2 occurrences of the digit 2 followed by the digits 57 is prime (see Example section).
a(25) > 2*10^5.
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LINKS
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EXAMPLE
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2 is in this sequence because (74*10^2 + 313)/9 = 857 is prime.
Initial terms and associated primes:
a(1) = 0, 43;
a(2) = 2, 857;
a(3) = 6, 8222257;
a(4) = 15, 8222222222222257;
a(5) = 18, 8222222222222222257; etc.
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MATHEMATICA
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Select[Range[0, 100000], PrimeQ[(74*10^# + 313)/9] &]
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PROG
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(PARI) isok(k) = isprime((74*10^k + 313)/9); \\ Michel Marcus, Nov 12 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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