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A102996 Numbers n such that 4*10^n + 7*R_n + 2 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 1

%I

%S 2,4,26,1271,1412,3328,4676,47051,67664

%N Numbers n such that 4*10^n + 7*R_n + 2 is prime, where R_n = 11...1 is the repunit (A002275) of length n.

%C Also numbers n such that (43*10^n+11)/9 is prime.

%C a(10) > 10^5. - _Robert Price_, Jun 01 2015

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/4/47779.htm#prime">Prime numbers of the form 477...779</a>.

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

%F a(n) = A101733(n) + 1.

%t Do[ If[ PrimeQ[(43*10^n + 11)/9], Print[n]], {n, 0, 10000}]

%Y Cf. A002275, A101733.

%K more,nonn

%O 1,1

%A _Robert G. Wilson v_, Jan 17 2005

%E a(8) from Erik Branger, May 01 2013, submitted by _Ray Chandler_, Aug 16 2013

%E a(9) from _Robert Price_, Jun 01 2015

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Last modified September 19 15:08 EDT 2019. Contains 327198 sequences. (Running on oeis4.)