## Open thread

Tomorrow I’m going to be gone pretty much all day. Dartmouth is a nice, straight, three-hour shot up I-91, so I’ll head out in the morning, spend the afternoon, and return at night. No sense paying for lodging or anything if I don’t have to.

Anyhow, since I won’t be able to post, I thought I’d leave this thread open. What’s on your mind? Anything you’d like to see treated here? Just who the hell are you people, anyway?

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Who am I? I’m an IT guy rediscovering my mathematical roots (more details at my blog…) I am also an amateur astronomer and currently have a 17.5″ dobsonian.

I’ve been following your Group Theory posts – your post on Cosets really made a ‘light bulb’ come on for me. Especially the description – ‘A subgroup is sort of like a slice through a group. The cosets of a subgroup are like slices “parallel” to the group’.

I’m going to work through the ‘The First Isomorphism Theorem’ in conjunction with the relevant section of Artin’s Algebra when I get time. It looks like an interesting and important part of the theory.

Comment by jasonwer | February 19, 2007 |

I, too am using Artin’s text along with watching Professor Benedict Gross’ fine lecture series from Harvard. Have you seen watched any of it.

Comment by Joe Tursi | November 17, 2013 |

I’m a low-dimensional topology grad student at UCSB, but this year I’m teaching at Westmont college. I believe that I actually sat near you at some of the talks at the NE sectional meeting of the AMS last year at Dartmouth, though I don’t think we ever actually met. I’m enjoying your blog so far…

Comment by Manalive | February 19, 2007 |

I’m a PhD student in Jena, Germany, trying to work out what A-infinity algebras have to do with group cohomology, and blogging in my copious freetime; as well as during work.

Found you over the cafe, and liked the quality of your posts.

Comment by Mikael Johansson | February 19, 2007 |

[EDIT: removed apropos-of-nothing political comment with, I must say, rather divisive “fightin’ words”.]Comment by nemoforone | March 29, 2007 |

A bored undergrad on winter break digging through your archives…

Comment by Nick | December 23, 2008 |

Welcome! Hope you find something interesting.

Comment by John Armstrong | December 23, 2008 |

I’m a final year biochemistry student at the moment, hopefully going on to do a PhD in something on the chemistry side of my subject. I read a bit of your blog a while back, I only read the bits I’m interested in (which is mostly category theory), but got confused around “MacLane’s Coherence Theorem”.

I came back because I learned read an enriched category is (in “arrows structures and functors: The Categorical Imperative”, although they called it a V-Category). Before learning that I found hom sets, the Yoneda lemma etc. rather boring (not just when I read them on your blog, also in the two books on category theory I’ve been reading), so decided to just stick with the best book. Now hom sets are suddenly interesting.

But I have a question. Have you covered anywhere about how fields fit with category theory (I can’t find any in your archives). I see they are rings, so you can talk about them in terms of ring algebras over the category of sets and functions. To talk about division you have to go to the category of sets and partial functions though – it just doesn’t look nice. But I can see that I’m probably missing something really cool. If you haven’t then that’s my request, although my opinion shouldn’t be worth tuppence ’round here, as I could well stop reading before you start (finals!) or not understand the discussion because I haven’t read most of the last few years of archives.

Comment by PhiJ | January 19, 2009 |

I have to admit, I really haven’t done much with fields. Really the only thing I know how to do with much depth in field theory is basic Galois theory, and I had other things I wanted to do first. I’ll probably hit Galois theory at some point, sure, but not yet.

One thing I can note now, since you’re interested in category theory, is that there’s something called a “Galois correspondence”, which basically amounts to a pair of direction-reversing maps of posets with a certain other property. But we know that posets are categories, and a direction-reversing map of posets is a contravariant functor. It turns out the extra condition is basically that the two functors are adjoints, and then certain other nice things happen. Once you have this viewpoint set up, a lot of basic Galois theory amounts to setting up some definitions, and the interesting conclusions just fall out of the category theory.

As for division, though.. division is always weird. There’s even a pretty heavy paper out there just on dividing by three!

Comment by John Armstrong | January 19, 2009 |

Wow! That paper is probably the most interesting one I have ever read. Although I’m still expecting it to get technical and difficult soon, I am rather excited that I know how to divide by two!

Comment by PhiJ | January 20, 2009 |

Undergraduate student reading the algebra here as an explanation of what the professor is mumbling about during the lectures.

Comment by Tommi | March 4, 2009 |