%I #18 Jan 17 2019 13:44:06

%S 2,3,23,38,62,75,107,165,519,1809,2348,2561,5783,6447,11691,16035,

%T 17553

%N Indices of primes in sequence defined by A(0) = 91, A(n) = 10*A(n-1) + 11 for n > 0.

%C Numbers n such that (830*10^n - 11)/9 is prime.

%C Numbers n such that digit 9 followed by n >= 0 occurrences of digit 2 followed by digit 1 is prime.

%C Numbers corresponding to terms <= 519 are certified primes.

%C a(18) > 10^5. - Robert Price, Nov 03 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/9/92221.htm#prime">Prime numbers of the form 922...221</a>.

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

%F a(n) = A103093(n) - 1.

%e 92221 is prime, hence 3 is a term.

%t Select[Range[0, 100000], PrimeQ[(830*10^# - 11)/9] &] (* _Robert Price_, Nov 03 2015 *)

%o (PARI) a=91;for(n=0,1000,if(isprime(a),print1(n,","));a=10*a+11)

%o (PARI) for(n=0,1000,if(isprime((830*10^n-11)/9 ),print1(n,",")))

%Y Cf. A000533, A002275, A103093.

%K nonn,hard,more,less

%O 1,1

%A _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Nov 27 2004

%E More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

%E a(15)-a(17) from Kamada data by _Ray Chandler_, Apr 28 2015