A101574 - OEIS

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

%S 0,1,3,6,102,165,279,474,504,834,921,2017,4434,4458,7614,8557,17163,

%T 22800

%N Indices of primes in sequence defined by A(0) = 53, A(n) = 10*A(n-1) - 7 for n > 0.

%C Numbers n such that (470*10^n + 7)/9 is prime.

%C Numbers n such that digit 5 followed by n >= 0 occurrences of digit 2 followed by digit 3 is prime.

%C Numbers corresponding to terms <= 921 are certified primes.

%C a(19) > 10^5. - _Robert Price_, Jul 10 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/5/52223.htm#prime">Prime numbers of the form 522...223</a>.

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

%F a(n) = A103009(n) - 1.

%e 52223 is prime, hence 3 is a term.

%o (PARI) a=53;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a-7)

%o (PARI) for(n=0,1500,if(isprime((470*10^n+7)/9),print1(n,",")))

%Y Cf. A000533, A002275, A103009.

%K nonn,more

%O 1,3

%A _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 09 2004

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

%E a(17)-a(18) from Kamada data by _Ray Chandler_, Apr 30 2015