

A056681


Numbers k such that 40*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.


2



1, 3, 10, 27, 54, 93, 474, 2079, 4834, 5844, 12337, 20193, 49528, 56988, 71545, 77308
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OFFSET

1,2


COMMENTS

Also numbers k such that (4*10^(k+1)31)/9 is prime.


LINKS



FORMULA



MATHEMATICA

Do[ If[ PrimeQ[40*(10^n  1)/9 + 1], Print[n]], {n, 0, 7000}]


CROSSREFS



KEYWORD

hard,nonn


AUTHOR



EXTENSIONS



STATUS

approved



