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A101006
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Indices of primes in sequence defined by A(0) = 91, A(n) = 10*A(n-1) + 31 for n > 0.
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1
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OFFSET
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1,2
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COMMENTS
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Numbers n such that (850*10^n - 31)/9 is prime.
Numbers n such that digit 9 followed by n >= 0 occurrences of digit 4 followed by digit 1 is prime.
Some of the larger entries may only correspond to probable primes.
a(10) > 10^5. - Robert Price, Nov 07 2015
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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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Table of n, a(n) for n=1..9.
Makoto Kamada, Prime numbers of the form 944...441.
Index entries for primes involving repunits.
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FORMULA
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a(n) = A103097(n) - 1.
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EXAMPLE
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944444441 is prime, hence 7 is a term.
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MATHEMATICA
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Select[Range[0, 100000], PrimeQ[(850*10^# - 31)/9] &] (* Robert Price, Nov 07 2015 *)
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PROG
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(PARI) a=91; for(n=0, 3000, if(isprime(a), print1(n, ", ")); a=10*a+31)
(PARI) for(n=0, 3000, if(isprime((850*10^n-31)/9), print1(n, ", ")))
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CROSSREFS
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Cf. A000533, A002275, A103097.
Sequence in context: A175986 A296351 A349007 * A306632 A083461 A137029
Adjacent sequences: A101003 A101004 A101005 * A101007 A101008 A101009
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KEYWORD
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nonn,hard,more,less
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AUTHOR
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Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Nov 27 2004
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EXTENSIONS
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More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
a(7)-a(8) from Kamada data by Ray Chandler, Apr 28 2015
a(9) from Robert Price, Nov 07 2015
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STATUS
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approved
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