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%I #20 Sep 08 2022 08:45:16
%S 0,3,5,6,9,15,21,30,1314,2063,6149,8706,12251,18609,21629,41711,44807,
%T 45420
%N Indices of primes in sequence defined by A(0) = 71, A(n) = 10*A(n-1) + 31 for n > 0.
%C Numbers n such that (670*10^n - 31)/9 is prime.
%C Numbers n such that digit 7 followed by n >= 0 occurrences of digit 4 followed by digit 1 is prime.
%C Some of the larger entries may only correspond to probable primes.
%C a(19) > 10^5. - _Robert Price_, Sep 28 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/7/74441.htm#prime">Prime numbers of the form 744...441</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A103057(n) - 1.
%e 74441 is prime, hence 3 is a term.
%t Select[Range[0, 100000], PrimeQ[(670*10^# - 31)/9] &] (* _Robert Price_, Sep 28 2015 *)
%o (PARI) a=71;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a+31)
%o (PARI) for(n=0,1500,if(isprime((670*10^n-31)/9),print1(n,",")))
%o (Magma) [n: n in [0..1000] | IsPrime((670*10^n-31) div 9)]; // _Vincenzo Librandi_, Sep 29 2015
%Y Cf. A000533, A002275, A103057.
%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(13)-a(18) from Kamada data by _Ray Chandler_, Apr 30 2015
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