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%I #28 Jan 17 2019 13:44:05
%S 1,3,9,19,21,57,73,81,207,349,421,3811,3981,20923,23785,38851,56041,
%T 68503,74433
%N Indices of primes in sequence defined by A(0) = 77, A(n) = 10*A(n-1) - 13 for n > 0.
%C Numbers n such that (680*10^n + 13)/9 is prime.
%C Numbers n such that digit 7 followed by n >= 0 occurrences of digit 5 followed by digit 7 is prime.
%C Numbers corresponding to terms <= 3981 are certified primes.
%D Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
%H Patrick De Geest, <a href="http://www.worldofnumbers.com/deplat.htm#pdp757">PDP Reference Table - 757</a>.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/75557.htm#prime">Prime numbers of the form 755...557</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A082713(n) - 2.
%e 75557 is prime, hence 3 is a term.
%t Select[Range[0, 2000], PrimeQ[(680 10^# + 13) / 9] &] (* _Vincenzo Librandi_, Nov 03 2014
%o (PARI) a=77;for(n=0,1000,if(isprime(a),print1(n,","));a=10*a-13)
%o (PARI) for(n=0,1000,if(isprime((680*10^n+13)/9),print1(n,",")))
%Y Cf. A000533, A002275, A082713.
%K nonn,hard,more
%O 1,2
%A _Robert G. Wilson v_, Aug 18 2000
%E Additional comments from _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 03 2004
%E Edited by _N. J. A. Sloane_ at the suggestion of _Andrew S. Plewe_, Jun 05 2007
%E More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
%E Three more terms added from Table, a link added, and comments section updated by _Patrick De Geest_, Nov 02 2014
%E Edited by _Ray Chandler_, Nov 05 2014
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