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A133127
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Numbers k with the property that k followed by k 7's is prime.
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1
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1, 2, 4, 32, 122, 388, 1580, 2302, 3436, 3866, 16993
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OFFSET
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1,2
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COMMENTS
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The numbers formed by a(1) to a(8) are certified prime and those formed by a(9) and a(10) are prp's.
a(11) > 10000.
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LINKS
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EXAMPLE
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2 is a term since 277 is prime.
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MATHEMATICA
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Select[Range[1600], PrimeQ[#*10^# + 7*(10^# - 1)/9] &] (* Amiram Eldar, Jan 27 2021 *)
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PROG
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(PARI) isok(n) = { ss = Str(7*(10^n-1)/9); ispseudoprime(eval(concat(Str(n), ss))); } \\ Michel Marcus, Aug 09 2013
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CROSSREFS
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KEYWORD
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nonn,base,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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