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Rasmus
  • Member for 13 years, 11 months
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50 votes
3 answers
15k views

Why doesn't $0$ being a prime ideal in $\mathbb Z$ imply that $0$ is a prime number?

41 votes
7 answers
11k views

How to tell if some power of my integer matrix is the identity?

28 votes
9 answers
4k views

Reference for spectral sequences

27 votes
3 answers
2k views

How to compute the series $\sum_{n=0}^\infty q^{n^2}$?

24 votes
9 answers
7k views

"Why do I always get 1 when I keep hitting the square root button on my calculator?"

23 votes
2 answers
2k views

Is the closedness of the image of a Fredholm operator implied by the finiteness of the codimension of its image?

20 votes
1 answer
3k views

Homogeneous topological spaces

18 votes
3 answers
3k views

Is there an algebraic homomorphism between two Banach algebras which is not continuous?

16 votes
1 answer
6k views

Does rapid decay of Fourier coefficients imply smoothness?

16 votes
1 answer
723 views

What is the spectrum of the commutative C*-algebra I have constructed here?

16 votes
5 answers
3k views

Reference for "It is enough to specify a sheaf on a basis"?

14 votes
1 answer
317 views

What is the decategorification of a triangulated category?

13 votes
3 answers
4k views

How to compute homotopy classes of maps on the 2-torus?

12 votes
1 answer
2k views

Is there a trick to prove injectivity for maps out of tensor products?

12 votes
1 answer
1k views

Sanity check: Is every T-principal bundle over T trivial?

10 votes
2 answers
1k views

Is the group isomorphism problem decidable for abelian groups?

10 votes
1 answer
607 views

What is the commutative analogue of a $C^*$-subalgebra?

9 votes
1 answer
1k views

Does the Gelfand transformation on $\ell^1(\mathbb Z)$ possess a continuous inverse on its image?

9 votes
3 answers
449 views

Can we turn the functor "category ring" into a 2-functor in a natural way?

8 votes
2 answers
449 views

What is the kernel of the summation map from the direct sum to the sum?

7 votes
1 answer
1k views

Does this covariant functor really turn coproducts into products?

6 votes
2 answers
2k views

How can I find a basis for the kernel of a homomorphism on a free abelian group?

6 votes
1 answer
266 views

Does the ring of integers have the following property?

6 votes
2 answers
886 views

Why do differential forms and integrands have different transformation behaviours under diffeomorphisms?

6 votes
1 answer
364 views

Comparison of two convergence conditions for sequences of non-negative numbers

5 votes
1 answer
512 views

Is there a notion of basis for Banach spaces?

5 votes
1 answer
210 views

Is there a list of all connected $T_0$-spaces with 5 points?

5 votes
0 answers
177 views

Projective objects in compounded Abelian category

5 votes
0 answers
235 views

Non-trivial conditions for $\mathrm{Ext}^2(A,B)=0$?

5 votes
0 answers
311 views

Reference request: Indecomposable representations of posets