A056705 - OEIS

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.

0, 1, 11, 17, 773, 18155

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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^n-1)/9], Print[n]], {n, 0, 30070}]

CROSSREFS

KEYWORD

hard,nonn

AUTHOR

STATUS

approved