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In algebraic topology, many point-set topology problems arise. For example, the product of quotient maps need not be a quotient map; the smash product may not be associative; the canonical map $Z^{X\times Y}\cong(Z^Y)^X$ is not necessarily a homeomorphism...

It seems that many authors chooses to stay in the category of compactly generated (weak Hausdorff) spaces to remedy this. For example, in Chapter 5 of Algebraic Topology by J. P. May, he introduces this concept and states without proof some basic properties, and then he assumes all topological spaces are compactly generated in the remainder of the book.

On the other hand, some authors do not assume this. Then some restrictions are necessary. In this context local compactness frequently crops up.

Here are my questions:

  1. Which approach should I take, as a beginner in this subject who does not want to be overwhelmed by technicalities?
  2. I currently know nothing about compactly generated spaces. I really want to read the text by May, but he uses this throughout, making many of his assertions simply false for me without the necessary restrictions (for instance, a cofibration need not be closed). How should I deal with this problem?
  3. Are there any readable introductions (be it a book, an article, lectures notes, etc.) to compactly generated spaces that provides working knowledge for use in algebraic topology?

Thanks for any advice!

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  • $\begingroup$ Very carefully. (This snarky comment means that I have absolutely no idea how to answer, or even understand, these questions.) $\endgroup$ Commented Dec 17, 2018 at 14:22
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    $\begingroup$ This is a common problem and was a problem for me as a beginner many years ago. I have no good answer. People make the assumptions that make their theorems provable. You can start by reading Steenrod's "A Convenient Category..." paper. $\endgroup$
    – Randall
    Commented Dec 17, 2018 at 14:24
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    $\begingroup$ And if you are talking about JP May's A Concise Course in Algebraic Topology, that is an excellent book but not a great one to learn from: it's written in a sophisticated, non-basic manner. If you're just starting out, there are easier ways to learn. (With all due respect to JPM, who is a superb writer and my mathematical grandfather.) $\endgroup$
    – Randall
    Commented Dec 17, 2018 at 14:35
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    $\begingroup$ If you care mostly about "algebraic" in algebraic topology, stick with polyhedra. I can see that you don't but that's your own choice. $\endgroup$ Commented Dec 17, 2018 at 16:57
  • $\begingroup$ Steenrod's notes were already mentioned. I also commend Stricklands The Category of CGWH Spaces $\endgroup$
    – Card_Trick
    Commented Dec 18, 2018 at 21:58

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Some thoughts:

  1. You should learn to cope with different assumptions. This will take time; I'm sorry. You will not learn all this overnight. You will commonly come across CW restrictions, simplicial stuff, and the compactly-generated category. There is really no way around this, since all these categories have their own intrinsic advantages. Ease with these notions comes only with experience using them.

  2. See point 1. When you need to, step aside and prove what he says is obvious. Or, just assume it's all fine and keep reading. Sweat the technicalities when you need to, not when someone else does.

  3. Steenrod's paper A Convenient Category of Topological Spaces was one of the first to advocate for compactly-generated spaces. It actually contains proofs of many of the things you're asking about.

Michigan Math. J., Volume 14, Issue 2 (1967), 133-152.

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  • $\begingroup$ Steenrod's paper has some queries about it: see the history in ncatlab.org/nlab/show/convenient+category+of+topological+spaces I hope my remarks in my book "Topology and Groupoids" get the message over that the search for a category "adequate and convenient for all purposes of topology" still has a way to go! $\endgroup$ Commented Dec 17, 2018 at 14:54
  • $\begingroup$ Thank you for your insightful comments! I guess maybe I should just try to prove eveything May says then. My doubt is basically this: Not every author uses compactly generated spaces systematically. So would it be better if (as a beginner) I choose to stay in the usual category of topological spaces? I'll definitely look into that paper you've mentioned! $\endgroup$
    – Yuxiao Xie
    Commented Dec 17, 2018 at 15:00
  • $\begingroup$ I think Munkres' book Elements Of Algebraic Topology has careful proofs of the basic results in the standard and weak topologies, with good examples to show the differences. He starts early with Simplicial Complexes and then when CW complexes re considered, goes into more detail. $\endgroup$ Commented Dec 17, 2018 at 16:33

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