The absolute easiest, clearest, and most entertaining book on Category
Theory (which I highly recommend) is:
Conceptual Mathematics: A First Introduction to Categories (Paperback)
by F. William Lawvere and Stephen Hoel Schanuel, $25
It literally reads like a series of college lectures (called Article),
each followed by study sections (called Sessions), complete with fake
dialog with fake students asking the questions that are probably on your
mind while reading the Articles.
It has a tone so conversational that working mathematicians will run
screaming, but I have found it highly enlightening. With hardly a proff
in sight, this is definitely the starter book par excellence of Category
Theory.
Here is a random excerpt on page 25, a dialog between Chad and the
Professor:
PROFESSOR: Is this correct? Not quite, because we are supposed to draw
two maps, and the thing draw for h . g is not a map; one of the points
of the domain of h . g has been left without an assigned output. This
deficiency won't matter for the next step, because that information is
going to get lost anyhow, but it belongs in this step and it is
incorrect to omit it. Chad's trouble was that in drawing h . g, he
notices that the last arrow would be irrelevant to the composite (h . g)
. f, so he left it out.
CHAD: It seems the principle is like in multiplication, where the order
in which you do things doesn't matter; you get the same answer.
PROFESSOR: I am glad you mention order. Let me give you an example...
I defy you to be scared by this book. There are an obscene amount of
diagrams illustrating the points as well, along with numerous (and
redundant) exercises to make sure you are getting it all.
Dan Weston
Seth Gordon wrote:
Are Benjamin C. Pierce's _Types and Programming Languages_ and/or _Basic
Category Theory for Computer Scientists_ suitable for self-study?
(Do they have problem sets that can be checked by either looking up
answers in The Back of the Book, or by trying to compile/run some code
that the student writes?)
FWIW, my formal math education--well, all the formal math education that
I can remember at this point--stopped at freshman calculus plus one
semester of statistics.
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