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A101961
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Indices of primes in sequence defined by A(0) = 21, A(n) = 10*A(n-1) + 41 for n > 0.
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0
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OFFSET
| 1,2
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COMMENTS
| Numbers n such that (230*10^n - 41)/9 is prime.
Numbers n such that digit 2 followed by n >= 0 occurrences of digit 5 followed by digit 1 is prime.
Numbers corresponding to terms <= 434 are certified primes.
a(n) = A098930(n-1) - 1.
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REFERENCES
| Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
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LINKS
| Makoto Kamada, Factorizations of near-repdigit numbers.
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EXAMPLE
| 255551 is prime, hence 4 is a term.
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PROG
| (PARI) a=21; for(n=0, 1800, if(isprime(a), print1(n, ", ")); a=10*a+41)
(PARI) for(n=0, 1800, if(isprime((230*10^n-41)/9), print1(n, ", ")))
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CROSSREFS
| Cf. A000533, A002275, A098930.
a(n) = A098930(n) - 1.
Sequence in context: A113058 A066145 A095022 * A090847 A056984 A117556
Adjacent sequences: A101958 A101959 A101960 * A101962 A101963 A101964
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KEYWORD
| nonn,hard,more
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AUTHOR
| 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
| 3446 from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
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