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Search: id:A101966
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| A101966 |
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Indices of primes in sequence defined by A(0) = 21, A(n) = 10*A(n-1) + 61 for n > 0. |
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+0
1
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OFFSET
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1,2
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COMMENT
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Numbers n such that (250*10^n - 61)/9 is prime.
Numbers n such that digit 2 followed by n >= 0 occurrences of digit 7 followed by digit 1 is prime.
Numbers corresponding to terms <= 935 are certified primes.
a(n) = A098960(n-1) - 1.
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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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Makoto Kamada, Factorizations of near-repdigit numbers.
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EXAMPLE
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271 is prime, hence 1 is a term.
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MAPLE
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Do[If[PrimeQ[(250*10^n-61)/9], Print[n]], {n, 1, 3250}] (Steinerberger)
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PROGRAM
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(PARI) a=21; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+61)
(PARI) for(n=0, 1500, if(isprime((250*10^n-61)/9), print1(n, ", ")))
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CROSSREFS
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Cf. A000533, A002275, A098960.
a(n) = A098960(n) - 1.
Sequence in context: A100840 A140154 A073694 this_sequence A089574 A077207 A001589
Adjacent sequences: A101963 A101964 A101965 this_sequence A101967 A101968 A101969
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KEYWORD
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nonn,hard,more
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AUTHOR
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Klaus Brockhaus (klaus-brockhaus(AT)t-online.de) and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 23 2004
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EXTENSIONS
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a(7) from Stefan Steinerberger (stefan.steinerberger(AT)gmail.com), Jan 31 2006
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
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