%I #17 Jan 17 2019 13:44:06
%S 0,4,10,11,16,27,45,47,60,72,137,177,212,404,761,1052,1156,1426,2864,
%T 3147,3614,13446,48080
%N Indices of primes in sequence defined by A(0) = 71, A(n) = 10*A(n-1) + 81 for n > 0.
%C Numbers n such that (720*10^n - 81)/9 is prime.
%C Numbers n such that digit 7 followed by n >= 0 occurrences of digit 9 followed by digit 1 is prime.
%C Numbers corresponding to terms <= 761 are certified primes.
%C a(24) > 2*10^5. - _Robert Price_, Dec 19 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/7/79991.htm#prime">Prime numbers of the form 799...991</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A103068(n) - 1.
%e 799991 is prime, hence 4 is a term.
%t Select[Range[0, 300], PrimeQ[(720*10^# - 81)/9] &] (* _Robert Price_, Dec 19 2015 *)
%o (PARI) a=71;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a+81)
%o (PARI) for(n=0,1500,if(isprime((720*10^n-81)/9),print1(n,",")))
%Y Cf. A000533, A002275, A103068.
%K nonn,hard,more
%O 1,2
%A _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 03 2004
%E a(19) from Mohammed Bouayoun (Mohammed.Bouayoun(AT)sanef.com), Apr 13 2006
%E More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
%E a(22)-a(23) from Kamada data by _Ray Chandler_, Apr 30 2015