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Numbers k such that 70*R_k + 3, where R_k = 11...1 is the repunit (A002275) of length k.
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%I #28 Jul 04 2021 22:09:52

%S 0,1,2,4,8,11,14,20,263,382,2719,4493,21166,45824,55850,64567,70726

%N Numbers k such that 70*R_k + 3, where R_k = 11...1 is the repunit (A002275) of length k.

%C Also numbers k such that (7*10^(k+1)-43)/9 is prime.

%C a(18) > 10^5. - _Robert Price_, Nov 01 2014

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/77773.htm#prime">Prime numbers of the form 77...773</a>.

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

%F a(n) = A099420(n) - 1. - _Robert Price_, Nov 01 2014

%t Do[ If[ PrimeQ[70*(10^n - 1)/9 + 3], Print[n]], {n, 0, 5000}]

%Y Cf. A002275, A093165, A099420.

%K hard,nonn

%O 1,3

%A _Robert G. Wilson v_, Aug 10 2000

%E a(13)-a(17) derived from A099420 by _Robert Price_, Nov 01 2014