Derek Elkins
Jason Dusek wrote:
What about the part that reads:
The unique arrow f making this diagram commute is then correspondingly denoted f1 ∐ f2 or f1 ⊕ f2 or f1 + f2 or [f1, f2]
This would seem to say that [f,g] and f+g are the same thing -- but if I've understood Derek Elkins' remarks, the latter is the functorial action on arrows while the former is just the mediating arrow.
In my experience the functorial action of a functor on arrows almost always has the same symbol as the action on objects. Furthermore, I don't think I've ever seen an example where the symbol used for the action on objects was used for something other than the action on arrows. I do think I have seen examples when a different symbol was used. If A+B is used then f+g should be the action on arrows.
Admittedly wikipedia does violate this. Perhaps it's using conventions from some branch that I'm not familiar with. Maybe it is just silly.
Indeed. Using f1 ∐ f2 or f1 + f2 for the mediating arrow is confusing, to say the least. -- _jsn