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%I #16 Jan 17 2019 13:44:06
%S 1,2,85,133,184,343,395,475,833,1798,2146,2417,5215,5881,6215,13393,
%T 19745,66484
%N Indices of primes in sequence defined by A(0) = 51, A(n) = 10*A(n-1) + 31 for n > 0.
%C Numbers n such that (490*10^n - 31)/9 is prime.
%C Numbers n such that digit 5 followed by n >= 0 occurrences of digit 4 followed by digit 1 is prime.
%C a(19) > 10^5. - Robert Price, Jul 16 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/5/54441.htm#prime">Prime numbers of the form 544...441</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A103013(n+1) - 1.
%e 5441 is prime, hence 2 is a term.
%t Select[Range[0,100000],PrimeQ[(490*10^#-31)/9]&] (* _Robert Price_, Jul 16 2015 *)
%o (PARI) a=51;for(n=0,1000,if(isprime(a),print1(n,","));a=10*a+31)
%o (PARI) for(n=0,1000,if(isprime((490*10^n-31)/9),print1(n,",")))
%Y Cf. A000533, A002275, A103013.
%K nonn,more
%O 1,2
%A _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 09 2004
%E 3 more terms from _Ryan Propper_, Jun 16 2005
%E More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Apr 28 2007
%E a(16)-a(17) by _Ray Chandler_, Apr 24 2015
%E a(18) from _Robert Price_, Jul 16 2015
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