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Mathematics of Computation

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Estimating the counts of Carmichael and Williams numbers with small multiple seeds

Author: Zhenxiang Zhang
Journal: Math. Comp. 84 (2015), 309-337
MSC (2010): Primary 11Y16, 11Y35; Secondary 11Y11
Published electronically: April 8, 2014
MathSciNet review: 3266962
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Abstract: For a positive even integer $ L$, let $ \mathcal {P}(L)$ denote the set of primes $ p$ for which $ p-1$ divides $ L$ but $ p$ does not divide $ L$, let $ \mathcal {C}(L)$ denote the set of Carmichael numbers $ n$ where $ n$ is composed entirely of primes in $ \mathcal {P}(L)$ and where $ L$ divides $ n-1$, and let $ \mathcal {W}(L)\subseteq \mathcal {C}(L)$ denote the subset of Williams numbers, which have the additional property that $ p+1 \mid n+1$ for each prime $ p\mid n$. We study $ \vert\mathcal {C}(L)\vert$ and $ \vert\mathcal {W}(L)\vert$ for certain integers $ L$. We describe procedures for generating integers $ L$ that have more even divisors than any smaller positive integer, and we obtain certain numerical evidence to support the conjectures that $ \log _2\vert\mathcal {C}(L)\vert=2^{s(1+o(1))}$ and $ \log _2\vert\mathcal {W}(L)\vert=2^{s^{1/2-o(1)}}$ when such an ``even-divisor optimal'' integer $ L$ has $ s$ different prime factors. For example, we determine that $ \vert\mathcal {C}(735134400)\vert > 2\cdot 10^{111}$. Last, using a heuristic argument, we estimate that more than $ 2^{24}$ Williams numbers may be manufactured from a particular set of $ 1029$ primes, although we do not construct any explicit examples, and we describe the difficulties involved in doing so.

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Additional Information

Zhenxiang Zhang
Affiliation: Department of Mathematics, Anhui Normal University, 241000 Wuhu, Anhui, People’s Republic of China
Email: ahnu{\textunderscore}, ahnu{\textunderscore}

Keywords: Carmichael numbers (with small multiple seeds), Korselt's criterion, Erd\H{o}s's construction of Carmichael numbers, even-divisors-optimal numbers (EDONs), Williams numbers, algorithms
Received by editor(s): June 26, 2012
Received by editor(s) in revised form: January 6, 2013, April 8, 2013, and May 1, 2013
Published electronically: April 8, 2014
Additional Notes: The author was supported by the NSF of China, Grant 10071001.
Dedicated: Dedicated to Guy Robin on the occasion of his 75th birthday
Article copyright: © Copyright 2014 American Mathematical Society
The copyright for this article reverts to public domain 28 years after publication.

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