%I #18 Jan 17 2019 13:44:06
%S 0,2,3,5,8,11,15,33,128,242,251,528,4884,6362,12950,13187,29930,34887,
%T 49515,67430,77780
%N Indices of primes in sequence defined by A(0) = 97, A(n) = 10*A(n-1) + 17 for n > 0.
%C Numbers n such that (890*10^n - 17)/9 is prime.
%C Numbers n such that digit 9 followed by n >= 0 occurrences of digit 8 followed by digit 7 is prime.
%C Numbers corresponding to terms <= 528 are certified primes.
%C a(22) > 10^5. - _Robert Price_, Nov 14 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/9/98887.htm#prime">Prime numbers of the form 988...887</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A103109(n) - 1.
%e 9888887 is prime, hence 5 is a term.
%t Select[Range[0, 100000], PrimeQ[(890*10^# - 17)/9] &] (* _Robert Price_, Nov 14 2015 *)
%o (PARI) a=97;for(n=0,1000,if(isprime(a),print1(n,","));a=10*a+17)
%o (PARI) for(n=0,1000,if(isprime((890*10^n-17)/9),print1(n,",")))
%Y Cf. A000533, A002275, A103109.
%K nonn,hard,more
%O 1,2
%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(19) from Kamada data by _Ray Chandler_, Apr 29 2015
%E a(20)-a(21) from _Robert Price_, Nov 14 2015