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%I #25 Sep 08 2022 08:45:16
%S 0,2,11,266,842,6299,37991,54116,121241,121620
%N Indices of primes in sequence defined by A(0) = 19, A(n) = 10*A(n-1) - 1 for n > 0.
%C Numbers n such that (170*10^n + 1)/9 is prime.
%C Numbers n such that digit 1 followed by n >= 0 occurrences of digit 8 followed by digit 9 is prime.
%C Numbers corresponding to terms <= 842 are certified primes.
%C a(11) > 2*10^5. - _Robert Price_, Nov 16 2014
%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/1/18889.htm#prime">Prime numbers of the form 188...889</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A102945(n) - 1.
%e 1889 is prime, hence 2 is a term.
%p A102031:=n->`if`(isprime((170*10^n+1)/9), n, NULL): seq(A102031(n), n=0..10^3); # _Wesley Ivan Hurt_, Nov 16 2014
%t Select[Range[0, 10^3], PrimeQ[(170*10^# + 1)/9] &] (* _Wesley Ivan Hurt_, Nov 16 2014 *)
%o (PARI) a=19;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a-1)
%o (PARI) for(n=0,1500,if(isprime((170*10^n+1)/9),print1(n,",")))
%o (Magma) [n: n in [0..300] | IsPrime((170*10^n+1) div 9)]; // _Vincenzo Librandi_, Nov 17 2014
%Y Cf. A000533, A002275, A102945.
%K nonn,hard,more
%O 1,2
%A _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 28 2004
%E 6299 from Herman Jamke (hermanjamke(AT)fastmail.fm), Dec 22 2007
%E a(7)-a(10) added by _Max Alekseyev_, Dec 12 2011
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