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%I #14 Jan 17 2019 13:44:06
%S 2,2042,5966,39854
%N Indices of primes in sequence defined by A(0) = 49, A(n) = 10*A(n-1) - 61 for n > 0.
%C Numbers n such that (380*10^n + 61)/9 is prime.
%C Numbers n such that digit 4 followed by n >= 0 occurrences of digit 2 followed by digit 9 is prime.
%C Some of the larger entries may only correspond to probable primes.
%C Certified primality of number corresponding to term 2042 using Primo. - _Ryan Propper_, Jun 23 2005
%C a(5) > 10^5. - _Robert Price_, May 08 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/4/42229.htm#prime">Prime numbers of the form 422...229</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A102987(n+1) - 1.
%e 4229 is prime, hence 2 is a term.
%t Select[Range[0, 3000], PrimeQ[(380*10^# + 61)/9] &] (* _Robert Price_, May 08 2015 *)
%o (PARI) a=49;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a-61)
%o (PARI) for(n=0,1500,if(isprime((380*10^n+61)/9),print1(n,",")))
%Y Cf. A000533, A002275, A102987.
%K nonn,bref,hard,more
%O 1,1
%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), Apr 28 2007
%E a(4) from Kamada link by _Ray Chandler_, Apr 24 2015