A101525 Indices of primes in sequence defined by A(0) = 61, A(n) = 10*A(n-1) + 21 for n > 0.

0, 1, 3, 6, 17, 18, 22, 23, 33, 40, 55, 63, 83, 148, 271, 754, 1271, 2397, 2685, 4799, 5197, 6216, 8736, 12387, 12390, 19701, 42403

Offset

1,3

Comments

Numbers n such that (570*10^n - 21)/9 is prime.

Numbers n such that digit 6 followed by n >= 0 occurrences of digit 3 followed by digit 1 is prime.

Numbers corresponding to terms <= 754 are certified primes.

a(28) > 10^5. - Robert Price, Sep 10 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..27.

Makoto Kamada, Prime numbers of the form 633...331.

Index entries for primes involving repunits.

Formula

a(n) = A103032(n) - 1.

Example

631 is prime, hence 1 is a term.

Mathematica

Select[Range[0, 100000], PrimeQ[(570*10^# - 21)/9] &] (* Robert Price, Sep 10 2015 *)

Prog

(PARI) a=61; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+21)
(PARI) for(n=0, 1500, if(isprime((570*10^n-21)/9), print1(n, ", ")))

Crossrefs

Cf. A000533, A002275, A103032.

Sequence in context: A195996 A036050 A173877 * A139476 A063618 A217084

Adjacent sequences: A101522 A101523 A101524 * A101526 A101527 A101528

Keyword

nonn,hard,more

Author

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

Extensions

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

a(24)-a(27) from Kamada data by Ray Chandler, Apr 30 2015

Status

approved