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%I #22 Jun 07 2024 14:14:42
%S 1,2,4,32,122,388,1580,2302,3436,3866,16993,35263,42601,51251
%N Numbers k with the property that k followed by k 7's is prime.
%C The numbers formed by a(1) to a(8) are certified prime and those formed by a(9) and a(10) are prp's.
%C a(11) > 10000.
%e 2 is a term since 277 is prime.
%t Select[Range[1600], PrimeQ[#*10^# + 7*(10^# - 1)/9] &] (* _Amiram Eldar_, Jan 27 2021 *)
%o (PARI) isok(n) = { ss = Str(7*(10^n-1)/9); ispseudoprime(eval(concat(Str(n), ss)));} \\ _Michel Marcus_, Aug 09 2013
%Y Cf. A084428.
%K nonn,base,more
%O 1,2
%A _Ray G. Opao_, Sep 19 2007
%E a(11) from _Amiram Eldar_, Jan 27 2021
%E a(12)-a(14) from _Michael S. Branicky_, Jun 06 2024
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