A101076 Indices of primes in sequence defined by A(0) = 81, A(n) = 10*A(n-1) + 61 for n > 0.

4, 5, 184, 400, 479, 1318, 5645, 6028, 16823, 62386, 70637

Offset

1,1

Comments

Numbers n such that (790*10^n - 61)/9 is prime.

Numbers n such that digit 8 followed by n >= 0 occurrences of digit 7 followed by digit 1 is prime.

Numbers corresponding to terms <= 479 are certified primes.

a(12) > 10^5. - Robert Price, Oct 25 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..11.

Makoto Kamada, Prime numbers of the form 877...771.

Index entries for primes involving repunits.

Formula

a(n) = A103089(n+1) - 1.

Example

8777771 is prime, hence 5 is a term.

Mathematica

Select[Range[0, 100000], PrimeQ[(790*10^# - 61)/9] &] (* Robert Price, Oct 25 2015 *)

Prog

(PARI) a=81; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+61)
(PARI) for(n=0, 1500, if(isprime((790*10^n-61)/9), print1(n, ", ")))

Crossrefs

Cf. A000533, A002275, A103089.

Sequence in context: A073129 A270551 A041409 * A331578 A355705 A041745

Adjacent sequences: A101073 A101074 A101075 * A101077 A101078 A101079

Keyword

nonn,hard,more

Author

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

Extensions

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

a(9) from Kamada data by Ray Chandler, Apr 29 2015

a(10)-a(11) from Robert Price, Oct 25 2015

Status

approved