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A056259 Indices of primes in sequence defined by A(0) = 77, A(n) = 10*A(n-1) - 13 for n > 0. 1
1, 3, 9, 19, 21, 57, 73, 81, 207, 349, 421, 3811, 3981, 20923, 23785, 38851, 56041, 68503, 74433 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Numbers n such that (680*10^n + 13)/9 is prime.

Numbers n such that digit 7 followed by n >= 0 occurrences of digit 5 followed by digit 7 is prime.

Numbers corresponding to terms <= 3981 are certified primes.

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..19.

Patrick De Geest, PDP Reference Table - 757.

Makoto Kamada, Prime numbers of the form 755...557.

Index entries for primes involving repunits.

FORMULA

a(n) = A082713(n) - 2.

EXAMPLE

75557 is prime, hence 3 is a term.

MATHEMATICA

Select[Range[0, 2000], PrimeQ[(680 10^# + 13) / 9] &] (* Vincenzo Librandi, Nov 03 2014

PROG

(PARI) a=77; for(n=0, 1000, if(isprime(a), print1(n, ", ")); a=10*a-13)

(PARI) for(n=0, 1000, if(isprime((680*10^n+13)/9), print1(n, ", ")))

CROSSREFS

Cf. A000533, A002275, A082713.

Sequence in context: A043097 A084670 A002091 * A056682 A032670 A068677

Adjacent sequences:  A056256 A056257 A056258 * A056260 A056261 A056262

KEYWORD

nonn,hard,more

AUTHOR

Robert G. Wilson v, Aug 18 2000

EXTENSIONS

Additional comments from Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 03 2004

Edited by N. J. A. Sloane at the suggestion of Andrew Plewe, Jun 05 2007

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

Three more terms added from Table, a link added, and comments section updated by Patrick De Geest, Nov 02 2014

Edited by Ray Chandler, Nov 05 2014

STATUS

approved

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Last modified September 23 23:21 EDT 2018. Contains 315306 sequences. (Running on oeis4.)