%I #15 Jan 17 2019 13:44:06
%S 0,1,3,6,21,22,79,4426,31318,80149,91143
%N Indices of primes in sequence defined by A(0) = 41, A(n) = 10*A(n-1) + 11 for n > 0.
%C Numbers n such that (380*10^n - 11)/9 is prime.
%C Numbers n such that digit 4 followed by n >= 0 occurrences of digit 2 followed by digit 1 is prime.
%C Numbers corresponding to terms <= 79 are certified primes.
%C a(12) > 10^5. - _Robert Price_, May 08 2015
%D Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/4/42221.htm#prime">Prime numbers of the form 422...221</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A102984(n) - 1.
%e 421 is prime, hence 1 is a term.
%t Select[Range[0, 3000], PrimeQ[(380*10^# - 11)/9] &] (* _Robert Price_, May 08 2015 *)
%o (PARI) a=41;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a+11)
%o (PARI) for(n=0,1500,if(isprime((380*10^n-11)/9),print1(n,",")))
%Y Cf. A000533, A002275, A102984.
%K nonn,hard,more
%O 1,3
%A _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 14 2004
%E 4426 from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
%E a(9) from Erik Branger May 01 2013 by _Ray Chandler_, Apr 30 2015
%E a(10)-a(11) from _Robert Price_, May 08 2015