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A101723
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Indices of primes in sequence defined by A(0) = 41, A(n) = 10*A(n-1) + 21 for n > 0.
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0
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0, 1, 3, 6, 9, 11, 19, 23, 29, 41, 61, 187, 303, 339, 714, 803, 1039, 1886, 2078, 2119, 2259, 2422, 3318, 5597, 6071
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OFFSET
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1,3
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COMMENTS
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Numbers n such that (390*10^n - 21)/9 is prime.
Numbers n such that digit 4 followed by n >= 0 occurrences of digit 3 followed by digit 1 is prime.
Numbers corresponding to terms <= 803 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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Table of n, a(n) for n=1..25.
Makoto Kamada, Factorizations of near-repdigit numbers.
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EXAMPLE
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431 is prime, hence 1 is a term.
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PROG
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(PARI) a=41; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+21)
(PARI) for(n=0, 1500, if(isprime((390*10^n-21)/9), print1(n, ", ")))
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CROSSREFS
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Cf. A000533, A002275.
a(n) = A102988(n) - 1.
Sequence in context: A145283 A177800 A169924 * A113944 A198514 A103695
Adjacent sequences: A101720 A101721 A101722 * A101724 A101725 A101726
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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 14 2004
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EXTENSIONS
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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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