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A056259
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Indices of primes in sequence defined by A(0) = 77, A(n) = 10*A(n-1) - 13 for n > 0.
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1
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1, 3, 9, 19, 21, 57, 73, 81, 207, 349, 421, 3811, 3981, 20923, 23785, 38851, 56041, 68503, 74433
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OFFSET
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1,2
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COMMENTS
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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.
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REFERENCES
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Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
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LINKS
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FORMULA
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EXAMPLE
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75557 is prime, hence 3 is a term.
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MATHEMATICA
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Select[Range[0, 2000], PrimeQ[(680 10^# + 13) / 9] &] (* Vincenzo Librandi, Nov 03 2014
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PROG
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(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, ", ")))
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CROSSREFS
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KEYWORD
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nonn,hard,more
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AUTHOR
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EXTENSIONS
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Additional comments from Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 03 2004
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
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STATUS
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approved
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