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 A133127 Numbers k with the property that k followed by k 7's is prime. 1
 1, 2, 4, 32, 122, 388, 1580, 2302, 3436, 3866, 16993 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS 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. LINKS Table of n, a(n) for n=1..11. EXAMPLE 2 is a term since 277 is prime. MATHEMATICA Select[Range[1600], PrimeQ[#*10^# + 7*(10^# - 1)/9] &] (* Amiram Eldar, Jan 27 2021 *) PROG (PARI) isok(n) = { ss = Str(7*(10^n-1)/9); ispseudoprime(eval(concat(Str(n), ss))); } \\ Michel Marcus, Aug 09 2013 CROSSREFS Cf. A084428. Sequence in context: A009098 A192387 A320624 * A122693 A019060 A247014 Adjacent sequences: A133124 A133125 A133126 * A133128 A133129 A133130 KEYWORD nonn,base,more AUTHOR Ray G. Opao, Sep 19 2007 EXTENSIONS a(11) from Amiram Eldar, Jan 27 2021 STATUS approved

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Last modified September 26 04:23 EDT 2023. Contains 365652 sequences. (Running on oeis4.)