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 A056720 Numbers n such that 7*10^n + 3*R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n. 1

%I

%S 0,1,2,3,5,53,56,343,908,1079,2204,2379,9134,9371,9728,11039,11247,

%T 13275,16781,20138,75368,87863

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

%C Also numbers n such that (22*10^n-1)/3 is prime.

%C a(23) > 10^5. - _Robert Price_, Dec 26 2014

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/73333.htm#prime">Prime numbers of the form 733...33</a>.

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

%t Do[ If[ PrimeQ[ 7*10^n + 3*(10^n-1)/9], Print[n]], {n, 0, 5000}]

%Y Cf. A002275, A093675.

%K hard,nonn

%O 1,3

%A _Robert G. Wilson v_, Aug 11 2000

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

%E a(16)-a(22) from _Robert Price_, Dec 26 2014

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Last modified May 25 11:17 EDT 2019. Contains 323539 sequences. (Running on oeis4.)