A101971 - OEIS

%I #18 Jan 17 2019 13:44:06

%S 2,5,17,68,442,448,454,2458,4744,7972,14248,31709

%N Indices of primes in sequence defined by A(0) = 27, A(n) = 10*A(n-1) + 17 for n > 0.

%C Numbers n such that (260*10^n - 17)/9 is prime.

%C Numbers n such that digit 2 followed by n >= 0 occurrences of digit 8 followed by digit 7 is prime.

%C Numbers corresponding to terms <= 454 are certified primes.

%C a(13) > 10^5. - _Robert Price_, Mar 28 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/2/28887.htm#prime">Prime numbers of the form 288...887</a>.

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

%F a(n) = A102962(n) - 1.

%e 2887 is prime, hence 2 is a term.

%t Select[Range@ 1000, PrimeQ[(260*10^# - 17)/9] &] (* _Michael De Vlieger_, Mar 29 2015 *)

%o (PARI) a=27;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a+17)

%o (PARI) for(n=0,1500,if(isprime((260*10^n-17)/9),print1(n,",")))

%Y Cf. A000533, A002275, A102962.

%K nonn,hard,more

%O 1,1

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

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

%E a(11)-a(12) derived from A102962 by _Robert Price_, Mar 28 2015