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A101525 Indices of primes in sequence defined by A(0) = 61, A(n) = 10*A(n-1) + 21 for n > 0.

%I A101525 #15 Jan 17 2019 13:44:06
%S A101525 0,1,3,6,17,18,22,23,33,40,55,63,83,148,271,754,1271,2397,2685,4799,
%T A101525 5197,6216,8736,12387,12390,19701,42403
%N A101525 Indices of primes in sequence defined by A(0) = 61, A(n) = 10*A(n-1) + 21 for n > 0.
%C A101525 Numbers n such that (570*10^n - 21)/9 is prime.
%C A101525 Numbers n such that digit 6 followed by n >= 0 occurrences of digit 3 followed by digit 1 is prime.
%C A101525 Numbers corresponding to terms <= 754 are certified primes.
%C A101525 a(28) > 10^5. - _Robert Price_, Sep 10 2015
%D A101525 Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
%H A101525 Makoto Kamada, <a href="https://stdkmd.net/nrr/6/63331.htm#prime">Prime numbers of the form 633...331</a>.
%H A101525 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F A101525 a(n) = A103032(n) - 1.
%e A101525 631 is prime, hence 1 is a term.
%t A101525 Select[Range[0, 100000], PrimeQ[(570*10^# - 21)/9] &] (* _Robert Price_, Sep 10 2015 *)
%o A101525 (PARI) a=61;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a+21)
%o A101525 (PARI) for(n=0,1500,if(isprime((570*10^n-21)/9),print1(n,",")))
%Y A101525 Cf. A000533, A002275, A103032.
%K A101525 nonn,hard,more
%O A101525 1,3
%A A101525 _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 06 2004
%E A101525 More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
%E A101525 a(24)-a(27) from Kamada data by _Ray Chandler_, Apr 30 2015