

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
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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^n1)/9); ispseudoprime(eval(concat(Str(n), ss))); } \\ Michel Marcus, Aug 09 2013


CROSSREFS



KEYWORD

nonn,base,more


AUTHOR



EXTENSIONS



STATUS

approved



