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%I #31 Jul 04 2021 22:10:41
%S 1,15,41,83,95,341,551,669,989,1223,6923,103703
%N Numbers k such that 10^k + 3*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also numbers k such that (4*10^k-1)/3 is prime.
%C a(13) > 850000 (from Kamada data). - _Robert Price_, Oct 19 2014
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/1/13333.htm#prime">Prime numbers of the form 133...33</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t Do[ If[ PrimeQ[ 10^n + 3*(10^n-1)/9], Print[n]], {n, 0, 30470}]
%o (PARI) for(k=1,1500,if(ispseudoprime(4*(10^k-1)/3+1),print1(k, ", "))) \\ _Hugo Pfoertner_, Jul 22 2020
%Y Cf. A002275, A093671, A097166, A259050.
%K nonn,hard,more
%O 1,2
%A _Robert G. Wilson v_, Aug 10 2000
%E a(12) from Kamada data by _Robert Price_, Oct 19 2014
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