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 A056707 Numbers k such that 4*10^k + 3*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k. 2

%I

%S 1,2,16,31,37,55,62,172,174,197,727,1246,1752,4318,4328,4930,6450,

%T 15206,24041,25321,33068,41484,51511

%N Numbers k such that 4*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 (13*10^k - 1)/3 is prime.

%C a(24) > 10^5. - _Robert Price_, Dec 21 2014

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/4/43333.htm#prime">Prime numbers of the form 433...33</a>.

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

%e 43 and 433 are prime, so 1 and 2 are terms of the sequence.

%t Do[ If[ PrimeQ[ 4*10^n + 3*(10^n-1)/9], Print[n]], {n, 0, 5000}]

%Y Cf. A002275, A093673.

%K hard,nonn

%O 1,2

%A _Robert G. Wilson v_, Aug 10 2000

%E 26044 from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

%E a(18)-a(23) from _Robert Price_, Dec 21 2014

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Last modified August 13 18:00 EDT 2022. Contains 356107 sequences. (Running on oeis4.)