A101540 Indices of primes in sequence defined by A(0) = 69, A(n) = 10*A(n-1) - 1 for n > 0.

4, 56, 500, 514, 626, 640, 724, 53110, 65330, 109672

Offset

1,1

Comments

Numbers n such that (620*10^n + 1)/9 is prime.

Numbers n such that digit 6 followed by n >= 0 occurrences of digit 8 followed by digit 9 is prime.

Numbers corresponding to terms <= 724 are certified primes.

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..10.

Makoto Kamada, Prime numbers of the form 688...889.

Index entries for primes involving repunits.

Formula

a(n) = A103047(n) - 1.

Example

688889 is prime, hence 4 is a term.

Prog

(PARI) a=69; for(n=0, 1000, if(isprime(a), print1(n, ", ")); a=10*a-1)
(PARI) for(n=0, 1000, if(isprime((620*10^n+1)/9), print1(n, ", ")))

Crossrefs

Cf. A000533, A002275, A103047.

Sequence in context: A204108 A077122 A020540 * A358882 A224181 A026740

Adjacent sequences: A101537 A101538 A101539 * A101541 A101542 A101543

Keyword

nonn,hard,more

Author

Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 06 2004

Extensions

a(8)-a(10) from Kamada data by Ray Chandler, Apr 30 2015

Status

approved