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A276545
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Numbers k such that (43*10^k - 421)/9 is prime.
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0
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2, 5, 7, 8, 11, 13, 25, 26, 61, 82, 131, 289, 377, 547, 845, 929, 1786, 5887, 6562, 10546, 28033, 33493, 150515, 205183
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OFFSET
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1,1
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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 7 followed by the digits 31 is prime (see Example section).
a(25) > 3*10^5.
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LINKS
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EXAMPLE
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4 is in this sequence because (43*10^4 - 421)/9 = 477731 is prime.
Initial terms and primes associated:
a(1) = 2, 431;
a(2) = 5, 477731;
a(3) = 7, 47777731;
a(4) = 8, 477777731;
a(5) = 11, 477777777731; etc.
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MATHEMATICA
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Select[Range[1, 100000], PrimeQ[(43*10^# - 421)/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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EXTENSIONS
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STATUS
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approved
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