cp's OEIS Frontend

A101715 Indices of primes in sequence defined by A(0) = 49, A(n) = 10*A(n-1) - 81 for n > 0.

%I A101715 #16 Jan 17 2019 13:44:06
%S A101715 1,3,4,7,8,27,190,195,2037,34413,39265,50578,94285,108411,130479,
%T A101715 178090,185354
%N A101715 Indices of primes in sequence defined by A(0) = 49, A(n) = 10*A(n-1) - 81 for n > 0.
%C A101715 Numbers n such that 40*10^n + 9 is prime.
%C A101715 Numbers n such that digit 4 followed by n >= 0 occurrences of digit 0 followed by digit 9 is prime.
%C A101715 Numbers corresponding to terms <= 195 are certified primes.
%C A101715 a(18) > 2*10^5. - Robert Price, May 24 2015
%D A101715 Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
%H A101715 Makoto Kamada, <a href="https://stdkmd.net/nrr/4/40009.htm#prime">Prime numbers of the form 400...009</a>.
%F A101715 a(n) = A101394(n+1) - 1.
%e A101715 40009 is prime, hence 3 is a term.
%t A101715 Select[Range[0, 1000], PrimeQ[40*10^# + 9] &] (* _Robert Price_, May 24 2015 *)
%o A101715 (PARI) a=49;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a-81)
%o A101715 (PARI) for(n=0,1500,if(isprime(40*10^n+9),print1(n,",")))
%Y A101715 Cf. A000533, A002275, A101394.
%K A101715 nonn,hard,more
%O A101715 1,2
%A A101715 _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 14 2004
%E A101715 2037 from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
%E A101715 a(10)-a(11) from Kamada data by _Ray Chandler_, Apr 30 2015
%E A101715 a(12)-a(17) from _Robert Price_, May 24 2015