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%I #22 Jan 17 2019 13:44:06
%S 0,1,3,24,57,147,372,420,1914,3745,16783,30049,60025,93345
%N Indices of primes in sequence defined by A(0) = 97, A(n) = 10*A(n-1) - 23 for n > 0.
%C Numbers n such that (850*10^n + 23)/9 is a prime.
%C Numbers n such that digit 9 followed by n >= 0 occurrences of digit 4 followed by digit 7 is a prime.
%C Numbers corresponding to terms <= 420 are certified primes.
%C a(15) > 10^5. - _Robert Price_, Nov 14 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/9/94447.htm#prime">Prime numbers of the form 944...447</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A103099(n) - 1.
%e 94447 is a prime, hence 3 is a term.
%t Flatten[Position[NestList[10#-23&,97,3800],_?PrimeQ]]-1 (* _Harvey P. Dale_, Apr 05 2015 *)
%t Select[Range[0, 100000], PrimeQ[(850*10^# + 23)/9] &] (* _Robert Price_, Nov 14 2015 *)
%o (PARI) a=97;for(n=0,1000,if(isprime(a),print1(n,","));a=10*a-23)
%o (PARI) for(n=0,1000,if(isprime((850*10^n + 23)/9),print1(n,",")))
%Y Cf. A000533, A002275, A103099.
%K nonn,hard,more,less
%O 1,3
%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(11)-a(12) from Kamada data by _Ray Chandler_, Apr 28 2015
%E a(13)-a(14) from _Robert Price_, Nov 14 2015
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