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%I #15 Jan 17 2019 13:44:06
%S 2,3,4,5,7,15,33,40,51,88,104,321,634,939,943,1130,1197,1742,1871,
%T 37913,41655,90095,99473
%N Indices of primes in sequence defined by A(0) = 91, A(n) = 10*A(n-1) + 51 for n > 0.
%C Numbers n such that (870*10^n - 51)/9 is prime.
%C Numbers n such that digit 9 followed by n >= 0 occurrences of digit 6 followed by digit 1 is prime.
%C Numbers corresponding to terms <= 1130 are certified primes.
%C a(24) > 10^5. - _Robert Price_, Jan 10 2017
%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/9/96661.htm#prime">Prime numbers of the form 966...661</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A103103(n) - 1.
%e 96661 is prime, hence 3 is a term.
%t Select[Range[0, 100000], PrimeQ[(290*10^# - 17)/3] &] (* _Robert Price_, Jan 10 2017 *)
%o (PARI) a=91;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a+51)
%o (PARI) for(n=0,1500,if(isprime((870*10^n-51)/9),print1(n,",")))
%Y Cf. A000533, A002275, A103103.
%K nonn,hard,more
%O 1,1
%A _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Nov 27 2004
%E More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
%E a(20)-a(21) from Kamada data by _Ray Chandler_, Apr 28 2015
%E a(22)-a(23) from _Robert Price_, Jan 10 2017
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