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%I #30 Jan 17 2019 13:44:05
%S 1,3,27,155,321,351,1211,1283,7983,15191,84771,119929,148859
%N Indices of primes in sequence defined by A(0) = 77, A(n) = 10*A(n-1) + 27 for n > 0.
%C Numbers n such that (720*10^n - 27)/9 is a prime.
%C Numbers n such that digit 7 followed by n >= 0 occurrences of digit 9 followed by digit 7 is a prime.
%C Numbers corresponding to terms <= 1283 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#pdp797">PDP Reference Table - 797</a>.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/79997.htm#prime">Prime numbers of the form 799...997</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A082716(n) - 2.
%e 797 is a prime, hence 1 is a term.
%o (PARI) a=77;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a+27)
%o (PARI) for(n=0,1500,if(isprime((720*10^n-27)/9),print1(n,",")))
%Y Cf. A000533, A002275, A082716.
%K nonn,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 08 2007
%E Update from De Geest link by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
%E a(11)=84771 from _Ray Chandler_, Jan 04 2011
%E a(12)=119929 from _Ray Chandler_, Apr 01 2011
%E a(13)=148859 from _Ray Chandler_, Apr 09 2011
%E Updated comments section by _Patrick De Geest_, Nov 02 2014
%E Edited by _Ray Chandler_, Nov 04 2014