A101061 Indices of primes in sequence defined by A(0) = 81, A(n) = 10*A(n-1) + 11 for n > 0.

1, 2, 4, 16, 46, 176, 8461, 36614

Offset

1,2

Comments

Numbers n such that (740*10^n - 11)/9 is prime.

Numbers n such that digit 8 followed by n >= 0 occurrences of digit 2 followed by digit 1 is prime.

Numbers corresponding to terms <= 176 are certified primes.

a(9) > 10^5. - Robert Price, Oct 13 2015

References

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

Links

Table of n, a(n) for n=1..8.

Makoto Kamada, Prime numbers of the form 822...221.

Index entries for primes involving repunits.

Formula

a(n) = A103074(n+1) - 1. - Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

Example

822221 is prime, hence 4 is a term.

Mathematica

Select[Range[0, 100000], PrimeQ[(740*10^# - 11)/9] &] (* Robert Price, Oct 13 2015 *)

Prog

(PARI) a=81; for(n=0, 1000, if(isprime(a), print1(n, ", ")); a=10*a+11)
(PARI) for(n=0, 1000, if(isprime((740*10^n-11)/9), print1(n, ", ")))

Crossrefs

Cf. A000533, A002275, A103074.

Sequence in context: A110128 A370658 A148279 * A148280 A112638 A077162

Adjacent sequences: A101058 A101059 A101060 * A101062 A101063 A101064

Keyword

nonn,hard,more

Author

Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Nov 30 2004

Extensions

8461 from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

a(8) from Erik Branger May 01 2013 by Ray Chandler, Apr 29 2015

Status

approved