%I #22 Jan 17 2019 13:44:07

%S 0,1,4,7,19,34,54,78,90,213,307,702,1456,2139,2214,3066,9300,16440,

%T 16894,28575,54051

%N Numbers n such that 4*10^n + 5*R_n - 2 is prime, where R_n = 11...1 is the repunit (A002275) of length n.

%C Also numbers n such that (41*10^n-23)/9 is prime.

%C a(22) > 10^5. _Robert Price_, May 08 2015

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/4/45553.htm#prime">Prime numbers of the form 455...553</a>.

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

%F a(n) = A101726(n-1) + 1.

%t Do[ If[ PrimeQ[(41*10^n - 23)/9], Print[n]], {n, 0, 10000}]

%Y Cf. A002275, A101726.

%K more,nonn

%O 1,3

%A _Robert G. Wilson v_, Jan 17 2005

%E Addition of a(17)-a(19) from Kamada data by _Robert Price_, Dec 10 2010

%E 0 added by _Arkadiusz Wesolowski_, Mar 10 2011

%E A(21) from _Robert Price_, May 08 2015