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A281989
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Numbers k such that 7*10^k + 61 is prime.
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0
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1, 2, 4, 6, 9, 14, 18, 20, 23, 24, 30, 61, 150, 209, 213, 226, 231, 364, 455, 3990, 15267, 24289, 28270, 94938, 155496
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OFFSET
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1,2
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COMMENTS
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For k>1, numbers such that the digit 7 followed by k-2 occurrences of the digit 0 followed by the digits 61 is prime (see Example section).
a(27) > 3*10^5.
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LINKS
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EXAMPLE
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4 is in this sequence because 7*10^4 + 61 = 70061 is prime.
Initial terms and primes associated:
a(1) = 1, 131;
a(2) = 2, 761;
a(3) = 4, 70061;
a(4) = 6, 7000061;
a(5) = 9, 7000000061; etc.
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MATHEMATICA
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Select[Range[0, 100000], PrimeQ[7*10^# + 61] &]
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PROG
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(PARI) isok(k) = isprime(7*10^k + 61); \\ 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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EXTENSIONS
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STATUS
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approved
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