Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #19 Jan 17 2019 13:44:06
%S 0,1,2,12,228,241,308,956,1472,1493,3181,3726,4176,23209,25718,32834,
%T 36989,103957
%N Indices of primes in sequence defined by A(0) = 41, A(n) = 10*A(n-1) - 9 for n > 0.
%C Numbers n such that 40*10^n + 1 is prime.
%C Numbers n such that digit 4 followed by n >= 0 occurrences of digit 0 followed by digit 1 is prime.
%D Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/4/40001.htm#prime">Prime numbers of the form 400...001</a>.
%F a(n) = A056806(n+1) - 1.
%e 4001 is prime, hence 2 is a term.
%t Select[Range[0, 1500], PrimeQ[4*10^# + 1] &] (* _Robert Price_, Mar 19 2015 *)
%o (PARI) a=41;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a-9)
%o (PARI) for(n=0,1500,if(isprime(40*10^n+1),print1(n,",")))
%Y Cf. A000533, A002275, A056806.
%K nonn,hard,more
%O 1,3
%A _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 14 2004
%E More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
%E Added missing term a(16)=32834 by _Robert Price_, Mar 19 2015