A101969 - OEIS

%I #18 Sep 08 2022 08:45:16

%S 1,11,26,43,79,118,130,274,314,875,1306,10894,17104,60716

%N Indices of primes in sequence defined by A(0) = 21, A(n) = 10*A(n-1) + 71 for n > 0.

%C Numbers n such that (260*10^n - 71)/9 is prime.

%C Numbers n such that digit 2 followed by n >= 0 occurrences of digit 8 followed by digit 1 is prime.

%C Numbers corresponding to terms <= 875 are certified primes.

%C a(15) > 10^5. - _Robert Price_, Mar 16 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/2/28881.htm#prime">Prime numbers of the form 288...881</a>.

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

%F a(n) = A102960(n) - 1.

%e 281 is prime, hence 1 is a term.

%t Select[Range[1000], PrimeQ[(260*10^# - 71)/9] &] (*_Robert Price_, Mar 17 2015*)

%o (PARI) a=21;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a+71)

%o (PARI) for(n=0,1500,if(isprime((260*10^n-71)/9),print1(n,",")))

%o (Magma) [n: n in [0..350] | IsPrime((260*10^n - 71) div 9)]; // _Vincenzo Librandi_, Mar 17 2015

%Y Cf. A000533, A002275, A102960.

%K nonn,hard,more

%O 1,2

%A _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 23 2004

%E a(12)-a(14) derived from A102960 by _Robert Price_, Mar 16 2015