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For questions requesting examples of a certain structure or phenomenon

14 votes

Request: A Serre fibration that is not a Dold fibration

You've already answered your own question, but here is another example. Let $f: \mathbb{Q}^\delta \to \mathbb{Q}$ be the obvious map from the rational numbers with the discrete topology to the ratio …
Chris Schommer-Pries's user avatar
9 votes

What category without initial object do you care about?

The bordism categories which arise in the study of topological quantum field theories are extremely interesting but don't have initial or terminal objects (except in degenerate cases). These categorie …
11 votes

Fundamental group of the line with the double origin.

You can compute that the fundamental group is Z by considering the fundamental groupoid. The main reason that Van Kampen's theorem isn't applicable in this case is due to connectivity. This can be cir …
Chris Schommer-Pries's user avatar
9 votes
2 answers
444 views

How weird can Modular Tensor Categories be over non-algebraically closed fields?

I don't see a reason to restrict to this case and I can imagine that there could be some very interesting examples, and hence interesting invariants, in other cases to. …
Chris Schommer-Pries's user avatar