login
Numbers n such that 6*10^n + 7*R_n - 4 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
1

%I #17 Jan 17 2019 13:44:07

%S 0,2,156,1055,1997,8930,9563,19560,26838,63857

%N Numbers n such that 6*10^n + 7*R_n - 4 is prime, where R_n = 11...1 is the repunit (A002275) of length n.

%C Also numbers n such that (61*10^n-43)/9 is prime.

%C a(11) > 10^5. - _Robert Price_, Sep 12 2015

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/6/67773.htm#prime">Prime numbers of the form 677...773</a>.

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

%F a(n) = A101535(n-1) + 1 for n>1.

%t Do[ If[ PrimeQ[(61*10^n - 43)/9], Print[n]], {n, 0, 10000}]

%Y Cf. A002275, A101535.

%K more,nonn

%O 1,2

%A _Robert G. Wilson v_, Jan 18 2005

%E a(8)-a(9) from Kamada data by _Robert Price_, Dec 14 2010

%E Inserted a(1)=0 by _Robert Price_, Sep 12 2015

%E a(10) from _Robert Price_, Sep 12 2015