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This column will publish short (from just a few paragraphs to ten or so pages), lively and intriguing computer-related mathematics vignettes. These vignettes or snapshots should illustrate ways in which computer environments have transformed the practice of mathematics or mathematics pedagogy. They could also include puzzles or brain-teasers involving the use of computers or computational theory. Snapshots are subject to peer review.This issue’s snapshot explores some generalizations of the definition of geometric reflection. Dynamic geometry tools can facilitate generalizations such as those obtained by relaxing the requirement that the reflection be through a straight line. The author compares the families of curves obtained by reflecting thru circular arcs with the curves generated in response to a physical problem proposed by Wittgenstein. He suggests that the strategy of generalizing definitions is a good avenue for bringing students quickly to the activity of doing mathematics. 相似文献
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Anniversaries and Dates
Aleksandr Nikolayevich Guz'on his 60th birthday 相似文献12.
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Given a finite subset
A{\cal A}
of an additive group
\Bbb G{\Bbb G}
such as
\Bbb Zn{\Bbb Z}^n
or
\Bbb Rn{\Bbb R}^n
, we are interested in efficient covering of
\Bbb G{\Bbb G}
by translates of
A{\cal A}
, and efficient packing of translates of
A{\cal A}
in
\Bbb G{\Bbb G}
. A set
S ì \Bbb G{\cal S} \subset {\Bbb G}
provides a covering if the translates
A + s{\cal A} + s
with
s ? Ss \in {\cal S}
cover
\Bbb G{\Bbb G}
(i.e., their union is
\Bbb G{\Bbb G}
), and the covering will be efficient if
S{\cal S}
has small density in
\Bbb G{\Bbb G}
. On the other hand, a set
S ì \Bbb G{\cal S} \subset {\Bbb G}
will provide a packing if the translated sets
A + s{\cal A} + s
with
s ? Ss \in {\cal S}
are mutually disjoint, and the packing is efficient if
S{\cal S}
has large density.
In the present part (I) we will derive some facts on these concepts when
\Bbb G = \Bbb Zn{\Bbb G} = {\Bbb Z}^n
, and give estimates for the minimal covering densities and maximal packing densities of finite sets
A ì \Bbb Zn{\cal A} \subset {\Bbb Z}^n
. In part (II) we will again deal with
\Bbb G = \Bbb Zn{\Bbb G} = {\Bbb Z}^n
, and study the behaviour of such densities under linear transformations. In part (III) we will turn to
\Bbb G = \Bbb Rn{\Bbb G} = {\Bbb R}^n
. 相似文献
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Jorge Almeida 《Journal of Pure and Applied Algebra》2008,212(3):486-499
We present an algorithm to compute the pointlike subsets of a finite semigroup with respect to the pseudovariety of all finite R-trivial semigroups. The algorithm is inspired by Henckell’s algorithm for computing the pointlike subsets with respect to the pseudovariety of all finite aperiodic semigroups. We also give an algorithm to compute -pointlike sets, where denotes the pseudovariety of all finite J-trivial semigroups. We finally show that, in contrast with the situation for , the natural adaptation of Henckell’s algorithm to computes pointlike sets, but not all of them. 相似文献