A101969 Indices of primes in sequence defined by A(0) = 21, A(n) = 10*A(n-1) + 71 for n > 0.

1, 11, 26, 43, 79, 118, 130, 274, 314, 875, 1306, 10894, 17104, 60716

Offset

1,2

Comments

Numbers n such that (260*10^n - 71)/9 is prime.

Numbers n such that digit 2 followed by n >= 0 occurrences of digit 8 followed by digit 1 is prime.

Numbers corresponding to terms <= 875 are certified primes.

a(15) > 10^5. - Robert Price, Mar 16 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..14.

Makoto Kamada, Prime numbers of the form 288...881.

Index entries for primes involving repunits.

Formula

a(n) = A102960(n) - 1.

Example

281 is prime, hence 1 is a term.

Mathematica

Select[Range[1000], PrimeQ[(260*10^# - 71)/9] &] (*Robert Price, Mar 17 2015*)

Prog

(PARI) a=21; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+71)
(PARI) for(n=0, 1500, if(isprime((260*10^n-71)/9), print1(n, ", ")))
(Magma) [n: n in [0..350] | IsPrime((260*10^n - 71) div 9)]; // Vincenzo Librandi, Mar 17 2015

Crossrefs

Cf. A000533, A002275, A102960.

Sequence in context: A047722 A161451 A100566 * A139576 A166137 A212018

Adjacent sequences: A101966 A101967 A101968 * A101970 A101971 A101972

Keyword

nonn,hard,more

Author

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

Extensions

a(12)-a(14) derived from A102960 by Robert Price, Mar 16 2015

Status

approved