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A101541
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Indices of primes in sequence defined by A(0) = 61, A(n) = 10*A(n-1) + 81 for n > 0.
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1
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0, 1, 2, 3, 13, 14, 22, 27, 53, 99, 271, 372, 402, 567, 638, 841, 968, 1254, 1258, 3046, 4837, 6388, 12754, 15141, 34942, 37651, 38107, 38685, 39383, 43392, 47279, 55029, 161191, 226478
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OFFSET
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1,3
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COMMENTS
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Numbers n such that (630*10^n - 81)/9 is prime.
Numbers n such that digit 6 followed by n >= 0 occurrences of digit 9 followed by digit 1 is prime.
Numbers corresponding to terms <= 968 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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69991 is prime, hence 3 is a term.
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MATHEMATICA
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Select[Range[0, 100000], PrimeQ[(630*10^# - 81)/9] &] (* Robert Price, Oct 14 2015 *)
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PROG
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(PARI) a=61; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+81)
(PARI) for(n=0, 1500, if(isprime((630*10^n-81)/9), print1(n, ", ")))
(Magma) [n: n in [0..500] | IsPrime((630*10^n-81) div 9)]; // Vincenzo Librandi, Oct 15 2015
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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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Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 06 2004
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EXTENSIONS
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More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
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STATUS
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approved
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