|
%I #13 Jan 17 2019 13:44:06
%S 0,1,3,6,9,11,19,23,29,41,61,187,303,339,714,803,1039,1886,2078,2119,
%T 2259,2422,3318,5597,6071,22047,25712,28643,43352
%N Indices of primes in sequence defined by A(0) = 41, A(n) = 10*A(n-1) + 21 for n > 0.
%C Numbers n such that (390*10^n - 21)/9 is prime.
%C Numbers n such that digit 4 followed by n >= 0 occurrences of digit 3 followed by digit 1 is prime.
%C a(30) > 10^5. - Robert Price, Mar 30 2015
%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/4/43331.htm#prime">Prime numbers of the form 433...331</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A102988(n) - 1.
%e 431 is prime, hence 1 is a term.
%t Select[Range[0, 100000], PrimeQ[(390*10^# - 21)/9] &]
%o (PARI) a=41;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a+21)
%o (PARI) for(n=0,1500,if(isprime((390*10^n-21)/9),print1(n,",")))
%Y Cf. A000533, A002275, A102988.
%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 a(26)-a(29) derived from A102988 by _Robert Price_, Mar 30 2015
|
