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%I #21 Jan 17 2019 13:44:05
%S 9,29,119,483,1485,1577,13671,13809,15093,72771,94211
%N Indices of primes in sequence defined by A(0) = 77, A(n) = 10*A(n-1) - 23 for n > 0.
%C Numbers n such that (670*10^n + 23)/9 is prime.
%C Numbers n such that digit 7 followed by n >= 0 occurrences of digit 4 followed by digit 7 is prime.
%C Numbers corresponding to terms <= 1577 are certified primes. For larger numbers see P. De Geest, PDP Reference Table.
%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#pdp747">PDP Reference Table - 747</a>.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/74447.htm#prime">Prime numbers of the form 744...447</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A082712(n) - 2.
%e 74444444447 is prime, hence 9 is a term.
%t Select[Range[0, 2000], PrimeQ[(670 10^# + 23) / 9] &] (* _Vincenzo Librandi_, Nov 03 2014 *)
%o (PARI) a=77;for(n=0,1600,if(isprime(a),print1(n,","));a=10*a-23)
%o (PARI) for(n=0,1600,if(isprime((670*10^n+23)/9),print1(n,",")))
%Y Cf. A000533, A002275, A082712.
%K nonn,hard,more
%O 1,1
%A _Robert G. Wilson v_, Aug 18 2000
%E Edited by _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 03 2004
%E More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
%E Two more terms added from PDP table and comments section updated by _Patrick De Geest_, Nov 02 2014
%E Edited by _Ray Chandler_, Nov 05 2014
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