%I #16 Jan 17 2019 13:44:06
%S 0,3,6,33,81,450,771,2553,6249,15136,20073,37377,53067
%N Indices of primes in sequence defined by A(0) = 79, A(n) = 10*A(n-1) - 41 for n > 0.
%C Numbers n such that (670*10^n + 41)/9 is prime.
%C Numbers n such that digit 7 followed by n >= 0 occurrences of digit 4 followed by digit 9 is prime.
%C Numbers corresponding to terms <= 771 are certified primes.
%C a(14) > 10^5. - Robert Price, Sep 27 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/7/74449.htm#prime">Prime numbers of the form 744...449</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A103059(n) - 1.
%e 74449 is prime, hence 3 is a term.
%t Select[Range[0, 100000], PrimeQ[(670*10^# + 41)/9] &] (* _Robert Price_, Sep 27 2015 *)
%o (PARI) a=79;for(n=0,1000,if(isprime(a),print1(n,","));a=10*a-41)
%o (PARI) for(n=0,1000,if(isprime((670*10^n+41)/9),print1(n,",")))
%Y Cf. A000533, A002275, A103059.
%K nonn,hard,more
%O 1,2
%A _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 03 2004
%E More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
%E a(10)-a(12) from Kamada data by _Ray Chandler_, Apr 30 2015
%E a(13) from _Robert Price_, Sep 27 2015