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%I #15 Jan 17 2019 13:44:06
%S 1,2,5,10,29,127,182,374,2158,2567,3368,24988,38809,59557,74873
%N Indices of primes in sequence defined by A(0) = 21, A(n) = 10*A(n-1) + 31 for n > 0.
%C Numbers n such that (220*10^n - 31)/9 is prime.
%C Numbers n such that digit 2 followed by n >= 0 occurrences of digit 4 followed by digit 1 is prime.
%C Numbers corresponding to terms <= 374 are certified primes.
%C a(16) > 10^5. - Robert Price, Apr 11 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/2/24441.htm#prime">Prime numbers of the form 244...441</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A102952(n) - 1.
%e 2441 is prime, hence 2 is a term.
%t Select[Range[0, 10000], PrimeQ[(220*10^# - 31)/9] &] (* _Robert Price_, Apr 11 2015 *)
%o (PARI) a=21;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a+31)
%o (PARI) for(n=0,1500,if(isprime((220*10^n-31)/9),print1(n,",")))
%Y Cf. A000533, A002275, A102952.
%K nonn,hard,more
%O 1,2
%A _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 23 2004
%E More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
%E a(12)-a(15) derived from A102952 by _Robert Price_, Apr 11 2015