Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #15 Jan 17 2019 13:44:06
%S 0,1,3,6,102,165,279,474,504,834,921,2017,4434,4458,7614,8557,17163,
%T 22800
%N Indices of primes in sequence defined by A(0) = 53, A(n) = 10*A(n-1) - 7 for n > 0.
%C Numbers n such that (470*10^n + 7)/9 is prime.
%C Numbers n such that digit 5 followed by n >= 0 occurrences of digit 2 followed by digit 3 is prime.
%C Numbers corresponding to terms <= 921 are certified primes.
%C a(19) > 10^5. - _Robert Price_, Jul 10 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/5/52223.htm#prime">Prime numbers of the form 522...223</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A103009(n) - 1.
%e 52223 is prime, hence 3 is a term.
%o (PARI) a=53;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a-7)
%o (PARI) for(n=0,1500,if(isprime((470*10^n+7)/9),print1(n,",")))
%Y Cf. A000533, A002275, A103009.
%K nonn,more
%O 1,3
%A _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 09 2004
%E More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
%E a(17)-a(18) from Kamada data by _Ray Chandler_, Apr 30 2015