A101154 Indices of primes in sequence defined by A(0) = 71, A(n) = 10*A(n-1) + 81 for n > 0.

0, 4, 10, 11, 16, 27, 45, 47, 60, 72, 137, 177, 212, 404, 761, 1052, 1156, 1426, 2864, 3147, 3614, 13446, 48080

Offset

1,2

Comments

Numbers n such that (720*10^n - 81)/9 is prime.

Numbers n such that digit 7 followed by n >= 0 occurrences of digit 9 followed by digit 1 is prime.

Numbers corresponding to terms <= 761 are certified primes.

a(24) > 2*10^5. - Robert Price, Dec 19 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..23.

Makoto Kamada, Prime numbers of the form 799...991.

Index entries for primes involving repunits.

Formula

a(n) = A103068(n) - 1.

Example

799991 is prime, hence 4 is a term.

Mathematica

Select[Range[0, 300], PrimeQ[(720*10^# - 81)/9] &] (* Robert Price, Dec 19 2015 *)

Prog

(PARI) a=71; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+81)
(PARI) for(n=0, 1500, if(isprime((720*10^n-81)/9), print1(n, ", ")))

Crossrefs

Cf. A000533, A002275, A103068.

Sequence in context: A106631 A120261 A310338 * A182943 A310339 A090070

Adjacent sequences: A101151 A101152 A101153 * A101155 A101156 A101157

Keyword

nonn,hard,more

Author

Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 03 2004

Extensions

a(19) from Mohammed Bouayoun (Mohammed.Bouayoun(AT)sanef.com), Apr 13 2006

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

a(22)-a(23) from Kamada data by Ray Chandler, Apr 30 2015

Status

approved