A101847 - OEIS

%I #14 Jan 17 2019 13:44:06

%S 1,2,5,16,19,44,56,77,118,136,508,709,1126,1517,1760,3085,3146,9925,

%T 19853,27841,46798,49681,68717,100541,106648,191072

%N Indices of primes in sequence defined by A(0) = 39, A(n) = 10*A(n-1) - 1 for n > 0.

%C Numbers n such that (350*10^n + 1)/9 is prime.

%C Numbers n such that digit 3 followed by n >= 0 occurrences of digit 8 followed by digit 9 is prime.

%C Numbers corresponding to terms <= 709 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/3/38889.htm#prime">Prime numbers of the form 388...889</a>.

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

%F a(n) = A102980(n) - 1.

%e 3889 is prime, hence 2 is a term.

%t Select[Range[0,1000],PrimeQ[(350*10^# + 1)/9]&]

%o (PARI) a=39;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a-1)

%o (PARI) for(n=0,1500,if(isprime((350*10^n+1)/9),print1(n,",")))

%Y Cf. A000533, A002275, A102980.

%K nonn,hard,more

%O 1,2

%A _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 20 2004

%E More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008

%E a(19)-a(26) derived from A102980 by _Robert Price_, Mar 17 2015