A101133 - OEIS

%I #19 Jan 17 2019 13:44:06

%S 0,3,15,24,28,40,46,87,1131,1569,3984,10608,12174,29368

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

%C Numbers n such that (650*10^n - 11)/9 is prime.

%C Numbers n such that digit 7 followed by n >= 0 occurrences of digit 2 followed by digit 1 is prime.

%C Some of the larger entries may only correspond to probable primes.

%C a(15) > 10^5. - _Robert Price_, Sep 23 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/7/72221.htm#prime">Prime numbers of the form 722...221</a>.

%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.

%F a(n) = A103052(n) - 1.

%e 72221 is prime, hence 3 is a term.

%t Select[Range[0, 100000], PrimeQ[(650*10^# - 11)/9] &] (* _Robert Price_, Sep 23 2015 *)

%o (PARI) a=71;for(n=0,1200,if(isprime(a),print1(n,","));a=10*a+11)

%o (PARI) for(n=0,1200,if(isprime((650*10^n-11)/9),print1(n,",")))

%Y Cf. A000533, A002275, A103052.

%K nonn,hard,more

%O 1,2

%A _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 03 2004

%E More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

%E a(12)-a(14) from Kamada data by _Ray Chandler_, Apr 29 2015