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%I #24 Jan 17 2019 13:44:06
%S 5,8,89,110,209,236,413,1191,1259,5835,6771,24860,52430
%N Indices of primes in sequence defined by A(0) = 91, A(n) = 10*A(n-1) + 71 for n > 0.
%C Numbers n such that (890*10^n - 71)/9 is prime.
%C Numbers n such that digit 9 followed by n >= 0 occurrences of digit 8 followed by digit 1 is prime.
%C Numbers corresponding to terms <= 413 are certified primes.
%C Certified primality of terms corresponding to 1191 and 1259 with Primo. - _Ryan Propper_, Jun 20 2005
%C a(14) > 10^5. - _Robert Price_, Nov 12 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/9/98881.htm#prime">Prime numbers of the form 988...881</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A103107(n+1) - 1.
%e 9888881 is prime, hence 5 is a term.
%t Select[Range[0, 100000], PrimeQ[(890*10^# - 71)/9] &] (* _Robert Price_, Nov 12 2015 *)
%o (PARI) a=91;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a+71)
%o (PARI) for(n=0,1500,if(isprime((890*10^n-71)/9),print1(n,",")))
%Y Cf. A000533, A002275, A103107.
%K nonn,hard,more
%O 1,1
%A _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Nov 27 2004
%E Two additional terms, corresponding to probable primes, from _Ryan Propper_, Jun 20 2005
%E Edited by _T. D. Noe_, Oct 30 2008
%E a(12) from Kamada data by _Ray Chandler_, Apr 29 2015
%E a(13) from _Robert Price_, Nov 12 2015
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