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Search: id:A047836
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| A047836 |
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"Nullwertzahlen" (or "inverse prime numbers"): n=p1*p2*p3*p4*p5*...*pk, where pi are primes with p1 <= p2 <= p3 <= p4 ...; then p1 = 2 and p1*p2*...*pi >= p(i+1) for all i < k. |
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+0
7
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| 2, 4, 8, 12, 16, 24, 32, 36, 40, 48, 56, 60, 64, 72, 80, 84, 96, 108, 112, 120, 128, 132, 144, 160, 168, 176, 180, 192, 200, 208, 216, 224, 240, 252, 256, 264, 280, 288, 300, 312, 320, 324, 336, 352, 360, 384, 392, 396, 400, 408, 416, 420, 432, 440, 448
(list; graph; listen)
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OFFSET
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1,1
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COMMENT
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Start with n and reach 2 by repeatedly either dividing by d where d <= the square root or by adding or subtracting 1. The division steps are free, but adding or subtracting 1 costs 1 point. The "value" of n (A047988) is the smallest cost to reach 2. Sequence gives numbers with value 0.
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REFERENCES
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Thomas Kantke, Mathematische Unterhaltungen, Spectrum der Wissenschaft, No. 4, 1993, pp. 11-13.
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LINKS
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T. D. Noe, Table of n, a(n) for n=1..1000
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FORMULA
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The number of a(n) <= x is conjectured to be about C * x / ln (x), where C = 0.61...
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EXAMPLE
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Starting at 24 we divide by 3, 2, then 2, reaching 2.
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CROSSREFS
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Cf. A047984-A047988.
Sequence in context: A006638 A001212 A118030 this_sequence A070173 A116882 A069519
Adjacent sequences: A047833 A047834 A047835 this_sequence A047837 A047838 A047839
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KEYWORD
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nonn,nice,easy
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AUTHOR
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Thomas Kantke (bytes.more(AT)ibm.net)
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EXTENSIONS
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More terms from David W. Wilson (davidwwilson(AT)comcast.net)
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