cp's OEIS Frontend

A056249 Indices of primes in sequence defined by A(0) = 11, A(n) = 10*A(n-1) + 71 for n > 0.

%I A056249 #20 Jan 17 2019 13:44:05
%S A056249 0,1,7,13,39,91,127,883,9423,14767,19257,31233
%N A056249 Indices of primes in sequence defined by A(0) = 11, A(n) = 10*A(n-1) + 71 for n > 0.
%C A056249 Numbers n such that (170*10^n - 71)/9 is prime.
%C A056249 Numbers n such that digit 1 followed by n >= 0 occurrences of digit 8 followed by digit 1 is prime.
%C A056249 Numbers corresponding to terms <= 883 are certified primes.
%D A056249 Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
%H A056249 Patrick De Geest, <a href="http://www.worldofnumbers.com/deplat.htm#pdp181">PDP Reference Table - 181</a>.
%H A056249 Makoto Kamada, <a href="https://stdkmd.net/nrr/1/18881.htm#prime">Prime numbers of the form 188...881</a>.
%H A056249 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F A056249 a(n) = A082702(n-1) - 2 for n > 1.
%e A056249 181 is prime, hence 1 is a term.
%t A056249 Select[Range[0, 2000], PrimeQ[(170 10^# - 71) / 9] &] (* _Vincenzo Librandi_, Nov 03 2014 *)
%o A056249 (PARI) a=11;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a+71)
%o A056249 (PARI) for(n=0,1500,if(isprime((170*10^n-71)/9),print1(n,",")))
%Y A056249 Cf. A000533, A002275, A068648, A082702.
%K A056249 nonn,hard,more
%O A056249 1,3
%A A056249 _Robert G. Wilson v_, Aug 18 2000
%E A056249 Additional comments from _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 28 2004
%E A056249 Edited by _N. J. A. Sloane_, Jun 15 2007
%E A056249 More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
%E A056249 Updated and added a link, by _Patrick De Geest_, Nov 02 2014
%E A056249 Edited by _Ray Chandler_, Nov 04 2014