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%I #14 Jan 17 2019 13:44:06
%S 1,7,1055,19834,20017,36463,81605
%N Indices of primes in sequence defined by A(0) = 87, A(n) = 10*A(n-1) - 13 for n > 0.
%C Numbers n such that (770*10^n + 13)/9 is prime.
%C Numbers n such that digit 8 followed by n >= 0 occurrences of digit 5 followed by digit 7 is prime.
%C Some of the larger entries may only correspond to probable primes.
%C a(8) > 10^5. - _Robert Price_, Oct 25 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/8/85557.htm#prime">Prime numbers of the form 855...557</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A103085(n) - 1.
%e 855555557 is prime, hence 7 is a term.
%t Select[Range[0, 100000], PrimeQ[(770*10^# + 13)/9] &] (* _Robert Price_, Oct 25 2015 *)
%o (PARI) a=87;for(n=0,2300,if(isprime(a),print1(n,","));a=10*a-13)
%o (PARI) for(n=0,2300,if(isprime((770*10^n+13)/9),print1(n,",")))
%Y Cf. A000533, A002275, A103085.
%K nonn,hard,more
%O 1,2
%A _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Nov 30 2004
%E a(4)-a(6) from Kamada data by _Ray Chandler_, Apr 29 2015
%E a(7) from _Robert Price_, Oct 25 2015
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