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%I #15 Jan 17 2019 13:44:06
%S 1,5,8,26,29,47,140,149,371,421,1096,3569,4606,5288,10733,11549,32603,
%T 42469,46310,67577
%N Indices of primes in sequence defined by A(0) = 51, A(n) = 10*A(n-1) + 61 for n > 0.
%C Numbers n such that (520*10^n - 61)/9 is prime.
%C Numbers n such that digit 5 followed by n >= 0 occurrences of digit 7 followed by digit 1 is prime.
%C Numbers corresponding to terms <= 421 are certified primes.
%C a(21) > 10^5. - _Robert Price_, Sep 05 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/5/57771.htm#prime">Prime numbers of the form 577...771</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A103019(n) - 1.
%e 571 is prime, hence 1 is a term.
%t Select[Range[0, 300], PrimeQ[(520*10^# - 61)/9] &] (* _Robert Price_, Sep 05 2015 *)
%o (PARI) a=51;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a+61)
%o (PARI) for(n=0,1500,if(isprime((520*10^n-61)/9),print1(n,",")))
%Y Cf. A000533, A002275, A103019.
%K nonn,more
%O 1,2
%A _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 09 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 30 2015
%E a(20) from _Robert Price_, Sep 05 2015
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