In this talk I explained the E8 root lattice and how it gives rise to the ‘octooctonionic projective plane’, a 128-dimensional manifold on which the compact Lie group called E8 acts as symmetries. I also discussed how some special root lattices give various notions of ‘integer’ for the real numbers, complex numbers, quaternions and octonions.
For more, read my paper Coxeter and Dynkin diagrams.
This was one of a series of lectures based on my column This Week’s Finds.
Azimuth 
Dear Prof. Baez, A while back, on your page https://johncarlosbaez.wordpress.com/2022/09/11/seminar-on-this-weeks-finds/ I read this:
I wanted to, but did/could not, attend this zoom session for 2 reasons: (1) you did not say the time-of-day (2) I had, and still have, no idea what that Passcode was/is. So, i would request:Please be clearer/more straightfoward in future to your projectve attendees,not all of whom are “in” with everything you write about. Best regards,[Dr.] Michael Oakes.
I’m sorry—if you had questions, I wish you’d asked.
I wrote that post before the talk times were known, but I added the time when it became known: 3 pm UK time on Thursdays. The last one for this year is on this Thursday, December 1st—see the details here.
If you Google “famous lemma category theory” the name will be clear—it was just a little test of motivation. The answer is “Yoneda”.