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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
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 A366365
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 March 28 04:13 EDT 2024. Contains 371235 sequences. (Running on oeis4.)