A101957 Indices of primes in sequence defined by A(0) = 21, A(n) = 10*A(n-1) + 31 for n > 0.

1, 2, 5, 10, 29, 127, 182, 374, 2158, 2567, 3368, 24988, 38809, 59557, 74873

Offset

1,2

Comments

Numbers n such that (220*10^n - 31)/9 is prime.

Numbers n such that digit 2 followed by n >= 0 occurrences of digit 4 followed by digit 1 is prime.

Numbers corresponding to terms <= 374 are certified primes.

a(16) > 10^5. - Robert Price, Apr 11 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..15.

Makoto Kamada, Prime numbers of the form 244...441.

Index entries for primes involving repunits.

Formula

a(n) = A102952(n) - 1.

Example

2441 is prime, hence 2 is a term.

Mathematica

Select[Range[0, 10000], PrimeQ[(220*10^# - 31)/9] &] (* Robert Price, Apr 11 2015 *)

Prog

(PARI) a=21; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+31)
(PARI) for(n=0, 1500, if(isprime((220*10^n-31)/9), print1(n, ", ")))

Crossrefs

Cf. A000533, A002275, A102952.

Sequence in context: A018377 A056363 A006404 * A047113 A239630 A239629

Adjacent sequences: A101954 A101955 A101956 * A101958 A101959 A101960

Keyword

nonn,hard,more

Author

Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 23 2004

Extensions

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

a(12)-a(15) derived from A102952 by Robert Price, Apr 11 2015

Status

approved