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%I #15 Jan 17 2019 13:44:07
%S 0,2,4,6,10,11,27,35,383,628,719,1397,2001,2621,6456,19304,38507,
%T 46454,95975
%N Indices of primes in sequence defined by A(0) = 19, A(n) = 10*A(n-1) - 21 for n > 0.
%C Numbers n such that (150*10^n + 21)/9 is prime.
%C Numbers n such that digit 1 followed by n >= 0 occurrences of digit 6 followed by digit 9 is prime.
%C Numbers corresponding to terms <= 719 are certified primes.
%C a(20) > 10^5. - _Robert Price_, Nov 15 2014
%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/1/16669.htm#prime">Prime numbers of the form 166...669</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F a(n) = A102941(n) - 1. - _Robert Price_, Nov 15 2014
%e 1669 is prime, hence 2 is a term.
%o (PARI) a=19;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a-21)
%o (PARI) for(n=0,1500,if(isprime((150*10^n+21)/9),print1(n,",")))
%Y Cf. A000533, A002275, A102941.
%K nonn,hard,more
%O 1,2
%A _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 28 2004
%E More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
%E a(16)-a(19) derived from A102941 by _Robert Price_, Nov 15 2014
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