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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
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.
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
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 S. 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