cp's OEIS Frontend

A056688 Numbers k such that 70*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

%I A056688 #25 Jul 04 2021 22:09:45
%S A056688 1,12,19,22,30,99,240,274,924,1066,1368,2064,7162,37962,91855,111705,
%T A056688 260197,271756,314563,348723
%N A056688 Numbers k such that 70*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056688 Also numbers k such that (7*10^(k+1)-61)/9 is prime.
%H A056688 Makoto Kamada, <a href="https://stdkmd.net/nrr/7/77771.htm">Prime numbers of the form 77...771</a>.
%H A056688 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A056688 a(n) = A099419(n) - 1.
%t A056688 Do[ If[ PrimeQ[70*(10^n - 1)/9 + 1], Print[n]], {n, 0, 7000}]
%Y A056688 Cf. A002275, A093176, A099419.
%K A056688 nonn
%O A056688 1,2
%A A056688 _Robert G. Wilson v_, Aug 10 2000
%E A056688 a(13) from Kamada link by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
%E A056688 a(14)-a(15) from Erik Branger, Dec 19 2009
%E A056688 a(16) from Erik Branger, Jan 29 2011
%E A056688 a(17) from Erik Branger, Nov 26 2011
%E A056688 a(18) from Erik Branger, Mar 05 2012
%E A056688 a(19)-a(20) from Erik Branger, Sep 23 2016