

A101969


Indices of primes in sequence defined by A(0) = 21, A(n) = 10*A(n1) + 71 for n > 0.


1



1, 11, 26, 43, 79, 118, 130, 274, 314, 875, 1306, 10894, 17104, 60716
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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. 462467.


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^n71)/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.rwthaachen.de), Dec 23 2004


EXTENSIONS

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


STATUS

approved



