A101150 - OEIS

%I #16 Jan 17 2019 13:44:06

%S 0,1,2,5,7,17,18,20,31,47,64,71,98,119,200,307,716,967,1343,2131,3566,

%T 3967,8326,10597,12464,15874,18894,28610,29417,83692

%N Indices of primes in sequence defined by A(0) = 79, A(n) = 10*A(n-1) - 21 for n > 0.

%C Numbers n such that (690*10^n + 21)/9 is prime.

%C Numbers n such that digit 7 followed by n >= 0 occurrences of digit 6 followed by digit 9 is prime.

%C Numbers corresponding to terms <= 967 are certified primes.

%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/7/76669.htm#prime">Prime numbers of the form 766...669</a>.

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

%F a(n) = A103065(n) - 1.

%e 7669 is prime, hence 2 is a term.

%t Select[Range[0, 100000], PrimeQ[(690*10^# + 21)/9] &] (* _Robert Price_, Oct 08 2015 *)

%o (PARI) a=79;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a-21)

%o (PARI) for(n=0,1500,if(isprime((690*10^n+21)/9),print1(n,",")))

%Y Cf. A000533, A002275, A103065.

%K nonn,hard,more

%O 1,3

%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(28)-a(29) from Kamada data by _Ray Chandler_, Apr 30 2015

%E a(30) from _Robert Price_, Oct 08 2015