A101728 Indices of primes in sequence defined by A(0) = 49, A(n) = 10*A(n-1) - 31 for n > 0.

5, 20, 21, 23, 50, 80, 131, 164, 620, 1449, 3423, 5978, 11525, 26979, 49232, 84926

Offset

1,1

Comments

Numbers n such that (410*10^n + 31)/9 is prime.

Numbers n such that digit 4 followed by n >= 0 occurrences of digit 5 followed by digit 9 is prime.

Numbers corresponding to terms <= 620 are certified primes.

a(17) > 10^5. - Robert Price, May 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..16.

Makoto Kamada, Prime numbers of the form 455...559.

Index entries for primes involving repunits.

Formula

a(n) = A102993(n) - 1.

Example

4555559 is prime, hence 5 is a term.

Mathematica

Select[Range[0, 1000], PrimeQ[(410*10^# + 31)/9] &] (* Robert Price, May 25 2015 *)

Prog

(PARI) a=49; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a-31)
(PARI) for(n=0, 1500, if(isprime((410*10^n+31)/9), print1(n, ", ")))

Crossrefs

Cf. A000533, A002275, A102993.

Sequence in context: A335555 A098047 A231276 * A053240 A034123 A338126

Adjacent sequences: A101725 A101726 A101727 * A101729 A101730 A101731

Keyword

nonn,hard,more

Author

Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 14 2004

Extensions

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

a(13)-a(15) from Kamada data by Ray Chandler, Apr 30 2015

a(16) from Robert Price, May 25 2015

Status

approved