cp's OEIS Frontend

A102008 Indices of primes in sequence defined by A(0) = 19, A(n) = 10*A(n-1) - 81 for n > 0.

%I A102008 #15 Jan 17 2019 13:44:07
%S A102008 0,1,2,3,8,17,21,44,48,55,68,145,201,271,2729,2840,4561,31809,43185,
%T A102008 48108,92690
%N A102008 Indices of primes in sequence defined by A(0) = 19, A(n) = 10*A(n-1) - 81 for n > 0.
%C A102008 Numbers n such that 10*10^n + 9 is prime.
%C A102008 Numbers n such that digit 1 followed by n >= 0 occurrences of digit 0 followed by digit 9 is prime.
%C A102008 Numbers corresponding to terms <= 271 are certified primes.
%C A102008 a(22) > 2*10^5. - _Robert Price_, Oct 11 2015
%D A102008 Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
%H A102008 Makoto Kamada, <a href="https://stdkmd.net/nrr/1/10009.htm#prime">Prime numbers of the form 100...009</a>.
%F A102008 a(n) = A088275(n) - 1.
%e A102008 1009 is prime, hence 2 is a term.
%t A102008 Select[Range[0, 200000], PrimeQ[10*10^# + 9] &] (* _Robert Price_, Oct 11 2015 *)
%o A102008 (PARI) a=19;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a-81)
%o A102008 (PARI) for(n=0,1500,if(isprime(10*10^n + 9),print1(n,",")))
%Y A102008 Cf. A000533, A002275, A088275.
%K A102008 nonn,hard,more
%O A102008 1,3
%A A102008 _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 28 2004
%E A102008 More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
%E A102008 a(21) from Kamada data by _Ray Chandler_, May 01 2015