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Numbers k with the property that k followed by k 7's is prime.
1

%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