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Indices of primes in sequence defined by A(0) = 27, A(n) = 10*A(n-1) - 13 for n > 0.
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%I #15 Jan 17 2019 13:44:06

%S 1,2,25,26,44,61,146,1276,1664,1978,2969,12772,17701,19127,22205,

%T 22385,25052

%N Indices of primes in sequence defined by A(0) = 27, A(n) = 10*A(n-1) - 13 for n > 0.

%C Numbers n such that (230*10^n + 13)/9 is prime.

%C Numbers n such that digit 2 followed by n >= 0 occurrences of digit 5 followed by digit 7 is prime.

%C a(18) > 10^5. - _Robert Price_, Feb 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/2/25557.htm#prime">Prime numbers of the form 255...557</a>.

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

%F a(n) = A099006(n) - 1.

%e 2557 is prime, hence 2 is a term.

%o (PARI) a=27;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a-13)

%o (PARI) for(n=0,1500,if(isprime((230*10^n+13)/9),print1(n,",")))

%Y Cf. A000533, A002275, A099006.

%K nonn,hard,more

%O 1,2

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

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

%E a(12)-a(17) derived from A099006 by _Robert Price_, Feb 16 2015