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A270831
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Numbers k such that (7*10^k + 71)/3 is prime.
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498
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1, 2, 3, 4, 5, 7, 23, 29, 37, 39, 40, 89, 115, 189, 227, 253, 301, 449, 533, 607, 969, 1036, 1207, 1407, 1701, 3493, 7147, 11342, 21638, 22327, 25575, 25648, 34079, 39974, 47719, 49913, 74729, 100737, 103531, 168067
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OFFSET
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1,2
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COMMENTS
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For n>1, numbers such that the digit 2 followed by n-2 occurrences of the digit 3 followed by the digits 57 is prime (see Example section).
a(41) > 2*10^5.
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LINKS
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EXAMPLE
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3 is in this sequence because (7*10^3 + 71)/3 = 2357 is prime.
Initial terms and primes associated:
a(1) = 1, 47;
a(2) = 2, 257;
a(3) = 3, 2357;
a(4) = 4, 23357;
a(5) = 5, 233357, etc.
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MATHEMATICA
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Select[Range[0, 100000], PrimeQ[(7*10^# + 71)/3] &]
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PROG
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(PARI) lista(nn) = {for(n=1, nn, if(ispseudoprime((7*10^n + 71)/3), print1(n, ", "))); } \\ Altug Alkan, Mar 23 2016
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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