A102021 Indices of primes in sequence defined by A(0) = 19, A(n) = 10*A(n-1) - 31 for n > 0.

0, 2, 3, 6, 11, 50, 86, 122, 197, 201, 536, 830, 1322, 2507, 2630, 5567, 6896, 7523, 9938, 10673, 11616, 13256, 14370, 20175, 36554, 36873, 40130, 43124, 58728, 97043

Offset

1,2

Comments

Numbers n such that (140*10^n + 31)/9 is prime.

Numbers n such that digit 1 followed by n >= 0 occurrences of digit 5 followed by digit 9 is prime.

Numbers corresponding to terms <= 830 are certified primes.

a(31) > 10^5. Robert Price, Feb 03 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..30.

Makoto Kamada, Prime numbers of the form 155...559.

Index entries for primes involving repunits.

Formula

a(n) = A102938(n) - 1.

Example

1559 is prime, hence 2 is a term.

Prog

(PARI) a=19; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a-31)
(PARI) for(n=0, 1500, if(isprime((140*10^n+31)/9), print1(n, ", ")))

Crossrefs

Cf. A000533, A002275, A102938.

Sequence in context: A369223 A065653 A275783 * A350911 A144641 A358237

Adjacent sequences: A102018 A102019 A102020 * A102022 A102023 A102024

Keyword

nonn,hard,more

Author

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

Extensions

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

a(20)-a(30) from Robert Price, Feb 03 2015

Status

approved