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%I #21 Sep 11 2020 22:26:19
%S 0,1,3,7,39,85,199,729,1459,23671,28629
%N Indices of primes in sequence defined by A(0) = 11, A(n) = 10*A(n-1) + 81 for n > 0.
%C Numbers n such that (180*10^n - 81)/9 is prime.
%C Numbers n such that digit 1 followed by n >= 0 occurrences of digit 9 followed by digit 1 is prime.
%C Numbers corresponding to terms <= 1459 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 Tony Crilly, <a href="https://doi.org/10.1017/mag.2020.43">A family of discrete spirals</a>, The Mathematical Gazette (2020) Vol. 104, Issue 560, 215-224.
%H Patrick De Geest, <a href="http://www.worldofnumbers.com/deplat.htm#pdp191">PDP Reference Table - 191</a>.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/1/19991.htm#prime">Prime numbers of the form 199...991</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A082703(n-1) - 2 for n > 1.
%e 191 is prime, hence 1 is a term.
%t Select[Range[0, 2000], PrimeQ[(180 10^# - 81) / 9] &] (* _Vincenzo Librandi_, Nov 03 2014 *)
%o (PARI) a=11;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a+81)
%o (PARI) for(n=0,1500,if(isprime((180*10^n-81)/9),print1(n,",")))
%Y Cf. A000533, A002275, A068649, A082703.
%K nonn,hard,more
%O 1,3
%A _Robert G. Wilson v_, Aug 18 2000
%E Additional comments from _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 28 2004
%E Edited by _N. J. A. Sloane_, Jun 15 2007
%E More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
%E Updated comments section and a link, by _Patrick De Geest_, Nov 02 2014
%E Edited by _Ray Chandler_, Nov 04 2014
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