## New answers tagged terminology

1
vote

### Where does the term "torsor" come from?

In the french school, un torseur sert à tordre, a torsor is used to twist.
More precisely, let $\eta$ be an object in a topos, and $G=\operatorname{Aut}(\eta)$.
If $\nu$ is a form of $\eta$ (another ...

5
votes

### Where does the term "torsor" come from?

There are three questions, but it's most sensible to answer them in reverse order.
(3) When and where did this piece of terminology originate?
Donu pointed out a reference to Giraud's thesis, and I ...

14
votes

Accepted

### What is this equivalence relation on topological spaces: there are bijective continuous maps in both directions

This relation was introduced (I don't know if for the first time) in the 1984 paper Bijectively related spaces I: Manifolds
by P. H. Doyle and J. G. Hocking. As the title indicates, two spaces that ...

4
votes

Accepted

### Name for generalization of trees to digraphs

Now I remember that I do know what these digraphs are called. A "directed tree" in the sense defined above is usually called a directed cactus. See for example Section 3 of "Remarks on ...

12
votes

### Why is the length spectrum called a spectrum?

I do not know of such an operator. But there is the following lovely theorem of Huber:
Theorem: Two compact hyperbolic surfaces have the same spectrum of the Laplacian if and only if they have the ...

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