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1 vote
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Intuition behind the diagonal lemma while proving Tarski's theorem about truth [duplicate]

Let $F$ be a first order logic theory with a set of axioms that are primitively recursive and are supposed to capture arithmetic (in particular, we can define a Gödel number. Let $\mathbb N$ be the st …
Asvin's user avatar
  • 7,746
11 votes
2 answers
3k views

Why the circle for Pontryagin duality? [duplicate]

For a locally compact group $G$, we define the Pontryagin dual as $\hat G = Hom(G,\mathbb T)$ where $\mathbb T$ is the circle group and the homomorphisms are continuous group maps. This duality has a …
Asvin's user avatar
  • 7,746
6 votes
0 answers
1k views

Relative cohomology in algebraic topology vs algebraic geometry

There is a notion of relative cohomology in both algebraic topology and algebraic geometry and the flexibility granted by this relative viewpoint is quite powerful in both cases. However, the two noti …
Asvin's user avatar
  • 7,746
42 votes
12 answers
7k views

Why is the definition of the higher homotopy groups the "right one"?

If someone asked me the question for the fundamental group, I would answer as follows: The connection to classification of covering spaces. The fundamental group of many spaces is an object of indep …