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A056249
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Indices of primes in sequence defined by A(0) = 11, A(n) = 10*A(n-1) + 71 for n > 0.
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2
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0, 1, 7, 13, 39, 91, 127, 883, 9423, 14767, 19257, 31233
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OFFSET
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1,3
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COMMENTS
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Numbers n such that (170*10^n - 71)/9 is prime.
Numbers n such that digit 1 followed by n >= 0 occurrences of digit 8 followed by digit 1 is prime.
Numbers corresponding to terms <= 883 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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181 is prime, hence 1 is a term.
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MATHEMATICA
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Select[Range[0, 2000], PrimeQ[(170 10^# - 71) / 9] &] (* Vincenzo Librandi, Nov 03 2014 *)
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PROG
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(PARI) a=11; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+71)
(PARI) for(n=0, 1500, if(isprime((170*10^n-71)/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 28 2004
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
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STATUS
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approved
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