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%I #12 Jan 17 2019 13:44:06
%S 2,8,14,20,26,86,179,1940,3959,4994,8576,23480,27854,80594
%N Indices of primes in sequence defined by A(0) = 27, A(n) = 10*A(n-1) - 23 for n > 0.
%C Numbers n such that (220*10^n + 23)/9 is prime.
%C Numbers n such that digit 2 followed by n >= 0 occurrences of digit 4 followed by digit 7 is prime.
%C Numbers corresponding to terms <= 179 are certified primes.
%C With alternating signs, expansion of cosh(atan(x)).
%C a(15) > 10^5. - _Robert Price_, Mar 15 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/2/24447.htm#prime">Prime numbers of the form 244...447</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A102954(n) - 1.
%e 2447 is prime, hence 2 is a term.
%t Select[Range[0, 100000], PrimeQ[(220*10^# + 23)/9] &] (* _Robert Price_, Mar 14 2015 *)
%o (PARI) a=27;for(n=0,2000,if(isprime(a),print1(n,","));a=10*a-23)
%o (PARI) for(n=0,2000,if(isprime((220*10^n+23)/9),print1(n,",")))
%Y Cf. A000533, A002275, A102954.
%K nonn,hard,more
%O 1,1
%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(14) derived from A102954 by _Robert Price_, Mar 15 2015
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