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A056660 Numbers n such that 20*R_n + 1 is a prime, where R_n = 11...1 is the repunit (A002275) of length n. 2

%I

%S 3,17,99,120,243,545,630,1493,2565,8087

%N Numbers n such that 20*R_n + 1 is a prime, where R_n = 11...1 is the repunit (A002275) of length n.

%C Also numbers n such that (2*10^(n+1)-11)/9 is a prime.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/2/22221.htm#prime">Prime numbers of the form 22...221</a>.

%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>

%F a(n) = A084832(n) - 1. - _Robert Price_, Jan 30 2015

%t Do[ If[ PrimeQ[ 20*(10^n - 1)/9 + 1 ], Print[n]], {n, 7000}]

%Y Cf. A002275, A084832, A091189 (corresponding primes).

%K nonn

%O 1,1

%A _Robert G. Wilson v_, Aug 09 2000

%E a(8) and a(9) from _Rick L. Shepherd_, Feb 22 2004

%E More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

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Last modified June 13 20:25 EDT 2021. Contains 345009 sequences. (Running on oeis4.)