A101076 - OEIS

%I #15 Jan 17 2019 13:44:06

%S 4,5,184,400,479,1318,5645,6028,16823,62386,70637

%N Indices of primes in sequence defined by A(0) = 81, A(n) = 10*A(n-1) + 61 for n > 0.

%C Numbers n such that (790*10^n - 61)/9 is prime.

%C Numbers n such that digit 8 followed by n >= 0 occurrences of digit 7 followed by digit 1 is prime.

%C Numbers corresponding to terms <= 479 are certified primes.

%C a(12) > 10^5. - Robert Price, Oct 25 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/8/87771.htm#prime">Prime numbers of the form 877...771</a>.

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

%F a(n) = A103089(n+1) - 1.

%e 8777771 is prime, hence 5 is a term.

%t Select[Range[0, 100000], PrimeQ[(790*10^# - 61)/9] &] (* _Robert Price_, Oct 25 2015 *)

%o (PARI) a=81;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a+61)

%o (PARI) for(n=0,1500,if(isprime((790*10^n-61)/9),print1(n,",")))

%Y Cf. A000533, A002275, A103089.

%K nonn,hard,more

%O 1,1

%A _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Nov 30 2004

%E 2 more terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

%E a(9) from Kamada data by _Ray Chandler_, Apr 29 2015

%E a(10)-a(11) from _Robert Price_, Oct 25 2015