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A281991
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Numbers k such that (598*10^k - 31)/9 is prime.
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0
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1, 4, 5, 7, 16, 17, 19, 28, 59, 68, 128, 307, 343, 368, 421, 448, 469, 691, 1949, 2506, 4868, 6914, 30025, 30955, 35522, 43388, 45716, 49999, 76309, 80149
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OFFSET
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1,2
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COMMENTS
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For k > 0, numbers k such that the digits 66 followed by k-1 occurrences of the digit 4 followed by the digit 1 is prime (see Example section).
a(31) > 2*10^5.
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LINKS
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EXAMPLE
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4 is in this sequence because (598*10^4 - 31)/9 = 664441 is prime.
Initial terms and associated primes:
a(1) = 1, 661;
a(2) = 4, 664441;
a(3) = 5, 6644441;
a(4) = 7, 664444441;
a(5) = 16, 664444444444444441; etc.
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MATHEMATICA
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Select[Range[0, 100000], PrimeQ[(598*10^# - 31)/9] &]
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PROG
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(PARI) isok(k) = isprime((598*10^k - 31)/9); \\ Michel Marcus, Feb 04 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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STATUS
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approved
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