A056658 - OEIS

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A056658

Numbers k such that 60*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 2, 3, 9, 17, 20, 21, 27, 42, 65, 120, 132, 177, 240, 453, 552, 1599, 2174, 2977, 3648, 7707, 8315, 10391, 12457, 21056, 26222, 48296, 64040, 84903, 92975, 95071

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OFFSET

1,2

COMMENTS

Also numbers k such that (2*10^(k+1)-17)/3 is prime.

LINKS

Table of n, a(n) for n=1..31.

Makoto Kamada, Prime numbers of the form 66...661.

Index entries for primes involving repunits

FORMULA

a(n) = A098088(n) - 1. - Robert Price, Aug 19 2014

MATHEMATICA