Comments (9)
It's true in general that the book could stand more exposition of the category-theoretic terminology that it uses, but that's a big enough change that I doubt it will happen. I do think (unsurprisingly, since I'm a category theorist) that there's value in including the terminology -- in your case, it motivated you to (re)learn it! But perhaps we could add a parenthetical here noting that "naturality of H" is nothing other than the just-proved Lemma 2.4.3.
from book.
I'm unsure why we need to whisker by (Hx)−1. Does (Hx)−1 not exist by Lemma 2.1.1? Then we can simply apply the fact that if p=q where p,q:x=y and r:y=z then p⋅r=q⋅r together with the equation from the first part of the proof and all the neccesary paths from Lemma 2.1.4. to arrive at the desired identity.
This is precisely a construction of whiskering. Theorem 2.1.6 defines it by direct path-induction on r, but you can also construct it out of other operations in the way you describe. (see below)
from book.
I like the use of the categorical language as long as it parallels the necessary and self contained information that comes before. I agree use of naturality should be relegated to a note or parenthesis (as you mentioned). A few paragraphs prior it is mentioned that a homotopy may be regarded as a natural isomorphism but no further context is given. For this reason I really do feel it is not sufficiently clear language for a proof.
from book.
So if I instead whiskered by
from book.
The operation of "whiskering by r" can be defined for all r at once by doing path induction on r, or it can be defined for any particular r by the operations you described.
from book.
Sorry, my brain must have been turned off. The correct answer is that this:
the fact that if p=q where p,q:x=y and r:y=z then p⋅r=q⋅r
is whiskering.
from book.
I think I was just starting to realize this! Thank you for clarifying!
from book.
Whiskering is admittedly a very cool name, but is it not a very desirable property of how identity and path composition should play together?
For any set with binary operation
Is this an example of some of the "coherence laws" that were mentioned following Lemma 2.1.4. and even the Lemma and Corollary in question?
from book.
It's true, it's also definable as
from book.
Related Issues (20)
- Assumption that `A // R` is a set HOT 4
- Rules for universes in Appendix A.2 are incomplete HOT 3
- identity type usage HOT 19
- Augment assumptions for Thm 5.4.4, 5.4.5, and 5.4.7 HOT 19
- Typos in proof of Lemma 10.3.12 HOT 5
- Use parentheses in the proof of Lemma 2.1.4(iii) HOT 3
- Cumulativity of the universe hierarchy HOT 4
- CI problem: "dubious ownership" HOT 1
- Provided Hashes in errata.pdf Not Found HOT 2
- Errata PDF unreadable
- Corollary 8.8.5 HOT 4
- Lemma 8.5.9 is missing a label
- Nightly builds pdfs are dead links HOT 8
- Exercise 7.3 could be made stronger
- Indexing of maps in fiber and exact sequences HOT 3
- proof-theoretic consistency in the introduction HOT 6
- cardinal numbers in the introduction HOT 5
- real numbers in the introduction HOT 7
- Switch to using truncated logic as default in the book HOT 2
- max and sup HOT 9
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from book.