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%I #19 Jan 17 2019 13:44:06
%S 0,2,18,29,39,41,44,69,306,537,1608,1667,1673,4491,4934,6111,12818,
%T 14078,16019,23004,50996,82869
%N Indices of primes in sequence defined by A(0) = 31, A(n) = 10*A(n-1) + 71 for n > 0.
%C Numbers n such that (350*10^n - 71)/9 is prime.
%C Numbers n such that digit 3 followed by n >= 0 occurrences of digit 8 followed by digit 1 is prime.
%C Numbers corresponding to terms <= 537 are certified primes.
%C a(23) > 10^5. - Robert Price, May 20 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/3/38881.htm#prime">Prime numbers of the form 388...881</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A102978(n) - 1.
%e 3881 is prime, hence 2 is a term.
%t #-1&/@(Flatten[Position[NestList[10#+71&,31,6200],_?(PrimeQ[#]&)]]) (* _Harvey P. Dale_, Aug 20 2011 *)
%o (PARI) a=31;for(n=0,1700,if(isprime(a),print1(n,","));a=10*a+71)
%o (PARI) for(n=0,1700,if(isprime((350*10^n-71)/9),print1(n,",")))
%Y Cf. A000533, A002275, A102978.
%K nonn,hard,more
%O 1,2
%A _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 20 2004
%E More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
%E a(17)-a(20) from Kamada data by _Ray Chandler_, May 01 2015
%E a(21)-a(22) from _Robert Price_, May 20 2015
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