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A272929
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Numbers k such that (8*10^k - 77)/3 is prime.
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0
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2, 4, 5, 6, 15, 18, 43, 45, 55, 60, 105, 128, 180, 207, 271, 479, 869, 1220, 1478, 1937, 4003, 4213, 5503, 9562, 11388, 13120, 34049, 47178, 156371, 271039
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OFFSET
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1,1
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COMMENTS
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For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 6 followed by the digits 41 is prime (see Example section).
a(31) > 3*10^5.
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LINKS
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EXAMPLE
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4 is in this sequence because (8*10^4 - 77)/3 = 26641 is prime.
Initial terms and primes associated:
a(1) = 2, 241;
a(2) = 4, 26641;
a(3) = 5, 266641;
a(4) = 6, 2666641;
a(5) = 15, 2666666666666641, etc.
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MATHEMATICA
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Select[Range[1, 100000], PrimeQ[(8*10^# - 77)/3] &]
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PROG
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(PARI) lista(nn) = {for(n=1, nn, if(ispseudoprime((8*10^n - 77)/3), print1(n, ", "))); } \\ Altug Alkan, May 11 2016
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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