An Introduction to Category Theory by Harold Simmons


By Harold Simmons

Class thought offers a common conceptual framework that has proved fruitful in matters as various as geometry, topology, theoretical machine technology and foundational arithmetic. here's a pleasant, easy-to-read textbook that explains the basics at a degree appropriate for newbies to the topic. starting postgraduate mathematicians will locate this booklet an outstanding creation to the entire fundamentals of type concept. It supplies the elemental definitions; is going throughout the a number of linked gadgetry, corresponding to functors, ordinary modifications, limits and colimits; after which explains adjunctions. the cloth is slowly constructed utilizing many examples and illustrations to light up the strategies defined. Over two hundred routines, with recommendations to be had on-line, aid the reader to entry the topic and make the booklet excellent for self-study. it will probably even be used as a advised textual content for a taught introductory path.

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Show that for each pair of Set-arrows A f 1 ✲ B ✲ A where the second represents the element a ∈ A, the composite 1 ✲ A f ✲ B represents the element f (a) ∈ B. 1). (a) Show that this category has a final object 1. (b) Show that for a presheaf A = (A, A) over S a global element 1 ✲ A is a kind of choice function for A. It ‘threads’ its way through the component sets A(s). Make precise the notion of ‘thread’. 5 Products and coproducts We all know how to form the cartesian product A×B of two sets A and B, the set of all ordered pairs (a, b) for a ∈ A and b ∈ B.

Thus g ◦ f = (l ◦ h) ◦ f = l ◦ (h ◦ f ) = l ◦ k is a more or less trivial calculation. However, it is more common to do this by chasing round the diagram and noting that certain composites are equal. Thus f ✲• f ✲• g ✲ • • = • • = • h ✲• ✲ ❄ ✲ l l k • • is what we trace out with our pencil and think whilst we are doing it. 36 Basic gadgetry Since this example is so trivial it doesn’t matter which method we use. For larger diagrams the chase is often easier to explain when talking to someone. This is a bit unfortunate since no-one has yet devised a method of writing down a diagram chase in an efficient manner.

The objects are the elements i, j, k, . . of S. Given a pair of objects i, j there is an arrow i ✲ j precisely when i ≤ j. Thus between any two objects there is at most one 32 Categories arrow. The existence of the arrow indicates a comparison between the objects. It is sometimes convenient to write (j, i) ✲ j i for this arrow. We have id i = (i, i) since i≤i (k, j) ◦ (j, i) = (k, i) since i≤j≤k⇒i≤k so the construction does give a category. Again this example looks rather feeble, but again we will add to it later to produce more interesting structures.

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