%I #15 Jan 17 2019 13:44:06
%S 0,1,3,6,23,171,172,217,438,2562,2781,2879,3714,4391,10819,14591,
%T 25053,27768,40167,180991,193906
%N Indices of primes in sequence defined by A(0) = 41, A(n) = 10*A(n-1) + 81 for n > 0.
%C Numbers n such that (450*10^n - 81)/9 is prime.
%C Numbers n such that digit 4 followed by n >= 0 occurrences of digit 9 followed by digit 1 is prime.
%C Numbers corresponding to terms <= 438 are certified primes.
%C a(22) > 2*10^5. - _Robert Price_, May 30 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/4/49991.htm#prime">Prime numbers of the form 499...991</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A103001(n) - 1.
%e 49991 is prime, hence 3 is a term.
%t Select[Range[0, 1000], PrimeQ[(450*10^# - 81)/9] &] (* _Robert Price_, May 30 2015 *)
%o (PARI) a=41;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a+81)
%o (PARI) for(n=0,1500,if(isprime((450*10^n-81)/9),print1(n,",")))
%Y Cf. A000533, A002275, A103001.
%K nonn,hard,more
%O 1,3
%A _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 14 2004
%E More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
%E a(17)-a(19) from Kamada data by _Ray Chandler_, May 01 2015
%E a(20)-a(21) from _Robert Price_, May 30 2015
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