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Indices of primes in sequence defined by A(0) = 21, A(n) = 10*A(n-1) + 41 for n > 0.
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%I #14 Jan 17 2019 13:44:06

%S 1,2,4,5,11,185,434,1745,3446,18797,70208

%N Indices of primes in sequence defined by A(0) = 21, A(n) = 10*A(n-1) + 41 for n > 0.

%C Numbers n such that (230*10^n - 41)/9 is prime.

%C Numbers n such that digit 2 followed by n >= 0 occurrences of digit 5 followed by digit 1 is prime.

%C Numbers corresponding to terms <= 434 are certified primes.

%C a(12) > 10^5. - _Robert Price_, Mar 04 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/2/25551.htm#prime">Prime numbers of the form 255...551</a>.

%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.

%F a(n) = A098930(n) - 1.

%e 255551 is prime, hence 4 is a term.

%o (PARI) a=21;for(n=0,1800,if(isprime(a),print1(n,","));a=10*a+41)

%o (PARI) for(n=0,1800,if(isprime((230*10^n-41)/9),print1(n,",")))

%Y Cf. A000533, A002275, A098930.

%K nonn,hard,more

%O 1,2

%A _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 23 2004

%E 3446 from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008

%E a(10)-a(11) from _Robert Price_, Mar 04 2015