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%I #17 Jan 17 2019 13:44:06
%S 0,1,5,24,68,252,299,383,433,720,821,882,2191,2648,3440,3444,8027,
%T 11654
%N Indices of primes in sequence defined by A(0) = 71, A(n) = 10*A(n-1) + 51 for n > 0.
%C Numbers n such that (690*10^n - 51)/9 is prime.
%C Numbers n such that digit 7 followed by n >= 0 occurrences of digit 6 followed by digit 1 is prime.
%C Numbers corresponding to terms <= 882 are certified primes.
%C a(19) > 10^5. - _Robert Price_, Oct 22 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/7/76661.htm#prime">Prime numbers of the form 766...661</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A103063(n+1) - 1.
%e 761 is prime, hence 1 is a term.
%t Select[Range[0, 100000], PrimeQ[(690*10^# - 51)/9] &] (* _Robert Price_, Oct 22 2015 *)
%o (PARI) a=71;for(n=0,1000,if(isprime(a),print1(n,","));a=10*a+51)
%o (PARI) for(n=0,1000,if(isprime((690*10^n-51)/9),print1(n,",")))
%Y Cf. A000533, A002275, A103063.
%K nonn,hard,more
%O 1,3
%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(18) from Kamada data by _Ray Chandler_, Apr 30 2015
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