

A056705


Numbers k such that 3*10^k + 7*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.


1




OFFSET

1,3


COMMENTS

Also numbers k such that (34*10^k  7)/9 is prime.


LINKS



MATHEMATICA

Do[ If[ PrimeQ[ 3*10^n + 7*(10^n1)/9], Print[n]], {n, 0, 30070}]


CROSSREFS



KEYWORD

hard,nonn


AUTHOR



STATUS

approved



