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%I #18 Jan 17 2019 13:44:06
%S 1,5,32,68,149,935,3134,5837,6989,20785,57137
%N Indices of primes in sequence defined by A(0) = 21, A(n) = 10*A(n-1) + 61 for n > 0.
%C Numbers n such that (250*10^n - 61)/9 is prime.
%C Numbers n such that digit 2 followed by n >= 0 occurrences of digit 7 followed by digit 1 is prime.
%C Numbers corresponding to terms <= 935 are certified primes.
%C a(12) > 10^5. - _Robert Price_, Feb 27 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/2/27771.htm#prime">Prime numbers of the form 277...771</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A098960(n) - 1.
%e 271 is prime, hence 1 is a term.
%t Do[If[PrimeQ[(250*10^n-61)/9], Print[n]], {n,1,3250}] (* _Stefan Steinerberger_, Jan 31 2006 *)
%o (PARI) a=21;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a+61)
%o (PARI) for(n=0,1500,if(isprime((250*10^n-61)/9),print1(n,",")))
%Y Cf. A000533, A002275, A098960.
%K nonn,hard,more
%O 1,2
%A _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 23 2004
%E a(7) from _Stefan Steinerberger_, Jan 31 2006
%E More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
%E a(10)-a(11) from _Robert Price_, Feb 27 2015
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