A101972 - OEIS

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

%S 0,6,7,10,13,18,31,40,52,60,132,156,238,1236,30253,34422,139288

%N Indices of primes in sequence defined by A(0) = 29, A(n) = 10*A(n-1) - 1 for n > 0.

%C Numbers n such that (260*10^n + 1)/9 is prime.

%C Numbers n such that digit 2 followed by n >= 0 occurrences of digit 8 followed by digit 9 is prime.

%C Numbers corresponding to terms <= 238 are certified primes.

%C a(18) > 2*10^5. - Robert Price, Jan 29 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/2/28889.htm#prime">Prime numbers of the form 288...889</a>.

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

%F a(n) = A102963(n+1) - 1.

%e 28888889 is prime, hence 6 is a term.

%t Flatten[Position[Table[FromDigits[Join[PadRight[{2},n,8],{9}]],{n,1300}],_?PrimeQ]]-1 (* _Harvey P. Dale_, Sep 22 2012 *)

%o (PARI) a=29;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a-1)

%o (PARI) for(n=0,1500,if(isprime((260*10^n+1)/9),print1(n,",")))

%Y Cf. A000533, A002275, A102963.

%K nonn,hard,more

%O 1,2

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

%E a(15)-a(17) derived from A102963 by _Robert Price_, Jan 29 2015