A further revised version of *Category Theory I* is now online. The main substantive changes are in the last few chapters. In particular, the short Chapter 24 on power objects is much improved.

There are also quite a few corrections of typos and thinkos — I should particularly thank Ruiting Jiang of the Queen’s College Oxford for comments.

What’s left to do before I paperback these notes? Add a final chapter on ‘the elementary theory of the category of sets’ as all the pieces are in place to cover that and an add an index. So to repeat what I said a couple of weeks ago, but with a tad more urgency, if you have been meaning to drop me a note with comments/suggestions/corrections, then now — yes, really *now* — is the time to do so!

Current versions of *Category Theory I* and *II* can be downloaded here.

AnonA tiny typo I’ve noticed: on page 75, line 18, the text reads “We saw. for example”, with a full stop instead of a comma.

And I think on page 91 it’s supposed to be “its mirror image (Y×X, π2, π1)”, not “(X×Y, π2, π1)”?

Peter SmithThanks for the first — even the tiniest corrections appreciated!

P. 91 is in fact correct as it stands. Maybe the use of “mirror image” is unhelpful. Look back at the original definition, if is a product of with , then you’ll see that counts as a product of with . Just apply that to the present case where we write for . (Maybe I should spell this out better!)

AnonWhoops, that makes sense. Thank you!