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Search: id:A130142
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| A130142 |
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Let f denote the map that replaces k by the concatenation of its proper divisors, written in decreasing order, each divisor being written in base 10 with its digits in reverse order. Then a(n) = prime reached when starting at 2n+1 and iterating f. |
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6
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| 1, 3, 5, 7, 3, 11, 13, 53, 17, 19, 73, 23, 5, 9343, 29, 31, 113
(list; graph; listen)
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OFFSET
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0,2
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COMMENT
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If 2n+1 is 1 or a prime, set a(n) = 2n+1. If no prime is ever reached, set a(n) = -1.
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EXAMPLE
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n = 13: 2n+1 = 27 has proper divisors 3 and 9, so we get 93, which has proper divisors 3 and 31, so we get 133.
Then 133 has proper divisors 7 and 19, so we get 917.
Then 917 has proper divisors 7 and 131, so we get 1317.
Then 1317 has proper divisors 3 and 439, so we get 9343, a prime, and a(13) = 9343.
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CROSSREFS
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Cf. A130139, A130140, A130141, A120716.
Adjacent sequences: A130139 A130140 A130141 this_sequence A130143 A130144 A130145
Sequence in context: A100029 A099984 A130141 this_sequence A130139 A101088 A134487
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KEYWORD
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base,more
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AUTHOR
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Adam L. Buchsbaum (alb(AT)research.att.com), Jul 30 2007, Aug 01 2007
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EXTENSIONS
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The value of a(17) is currently unknown.
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