A056249 Indices of primes in sequence defined by A(0) = 11, A(n) = 10*A(n-1) + 71 for n > 0.

0, 1, 7, 13, 39, 91, 127, 883, 9423, 14767, 19257, 31233

Offset: 1

Author

Robert G. Wilson v, Aug 18 2000

Keywords

• nonn
• hard
• more

Comments

Numbers n such that (170*10^n - 71)/9 is prime.

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

Numbers corresponding to terms <= 883 are certified primes.

Examples

			181 is prime, hence 1 is a term.
		

References

• Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.

Links

• Patrick De Geest, PDP Reference Table - 181.
• Makoto Kamada, Prime numbers of the form 188...881.
• Index entries for primes involving repunits.

Crossrefs

Cf. A000533, A002275, A068648, A082702.

Programs

Mathematica

Select[Range[0, 2000], PrimeQ[(170 10^# - 71) / 9] &] (* Vincenzo Librandi, Nov 03 2014 *)

PARI

a=11;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a+71)

PARI

for(n=0,1500,if(isprime((170*10^n-71)/9),print1(n,",")))

Formula

a(n) = A082702(n-1) - 2 for n > 1.

Extensions

Additional comments from Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 28 2004

Edited by N. J. A. Sloane, Jun 15 2007

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

Updated and added a link, by Patrick De Geest, Nov 02 2014

Edited by Ray Chandler, Nov 04 2014