Author Archives: limsup

Commutative Algebra 7

Posted on March 25, 2020 by limsup

Modules Having dipped our toes into algebraic geometry, we are back in commutative algebra. Next we would like to introduce “linear algebra” over a ring A. Most of the proofs should pose no difficulty to the reader so we will … Continue reading →

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Commutative Algebra 6

Posted on March 24, 2020 by limsup

Injective and Surjective Maps Proposition 1. Let be a morphism of closed sets, with corresponding . is injective if and only if is dense. is surjective if and only if is an embedding of V as a closed subspace of … Continue reading →

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Commutative Algebra 5

Posted on March 23, 2020 by limsup

Morphisms in Algebraic Geometry Next we study the “nice” functions between closed subspaces of . Definition. Suppose and are closed subsets. A morphism is a function which can be expressed as: for some polynomials . We also say f is a regular … Continue reading →

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Commutative Algebra 4

Posted on March 22, 2020 by limsup

More Concepts in Algebraic Geometry As before, k denotes an algebraically closed field. Recall that we have a bijection between radical ideals of and closed subsets of . The bijection reverses the inclusion so if and only if . Not too … Continue reading →

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Commutative Algebra 3

Posted on March 21, 2020 by limsup

Algebraic Geometry Concepts We have decided to introduce, at this early point, some basics of algebraic geometry in order to motivate the later concepts. In summary, algebraic geometry studies solutions to polynomial equations over a field. First we consider a … Continue reading →

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Commutative Algebra 2

Posted on March 20, 2020 by limsup

Radical of an Ideal In this installation, we will study more on ideals of a ring A. Definition. If is an ideal, its radical is defined by To fix ideas, again consider the case again. For the ideal (m) where , … Continue reading →

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Commutative Algebra 1

Posted on March 19, 2020 by limsup

More About Ideals Recall that we defined three operations on ideals: intersection, sum and product. We can take intersection and sum of any collection of ideals (even infinitely many of them), but we can only define the product of finitely many … Continue reading →

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Commutative Algebra 0

Posted on March 18, 2020 by limsup

We’re starting a new series on commutative algebra. This has been in the works for way too long, and eventually we just decided to push ahead with it anyway. Most of the articles will be short, and we’ll try to … Continue reading →

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Free Groups and Tiling

Posted on June 26, 2018 by limsup

Introduction Consider the following simple problem. Prove that the shape on the left cannot be completely tiled by 20 polygons of the types shown on the right. The solution is rather simple: colour the shape in the following manner. This … Continue reading →

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Solving Permutation-Based Puzzles

Posted on June 21, 2018 by limsup

Introduction In the previous article, we described the Schreier-Sims algorithm. Given a small subset which generates the permutation group G, the algorithm constructs a sequence such that for: we have a small generating set for each Specifically, via the Sims … Continue reading →

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