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pancini
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20 votes
3 answers
8k views

Every normal subgroup is the kernel of some homomorphism

14 votes
4 answers
12k views

Does same cardinality imply a bijection?

12 votes
2 answers
2k views

Intuition for Inclusion-Exclusion Principle

11 votes
1 answer
285 views

Prove addition law from duplication formula (for power series associated to elliptic genus)

9 votes
4 answers
2k views

Why do we say "radius" of convergence?

8 votes
1 answer
373 views

Is there a function $f\colon \Bbb R\to \Bbb R$ with uncountably many critical values?

8 votes
0 answers
108 views

Intuition for Freudenthal Suspension

6 votes
2 answers
342 views

Nilpotent elements of group algebra $\Bbb CG$

6 votes
1 answer
97 views

Show there is no group $G$ of order $240$ with $5$ conjugacy classes

6 votes
5 answers
5k views

What do we call a "function" which is not defined on part of its domain?

6 votes
1 answer
2k views

Why is the commutator group a subgroup?

6 votes
1 answer
107 views

If $M^{2n}$ is oriented, then $H_n(M)\to H_n(M,\partial M)$ can be identified with the matrix of the intersection form

5 votes
2 answers
406 views

Collecting proofs $SO(3)\cong\Bbb R P^3$

5 votes
0 answers
144 views

Possible error in McCleary's guide to spectral sequences

5 votes
2 answers
223 views

Derangements with extra chairs

5 votes
1 answer
363 views

"Visualizing'' rotation in even dimensions

4 votes
1 answer
91 views

Show that $\Bbb Z[\alpha,4/\alpha]/(2)$ is a direct product of three domains, where $\alpha^3+\alpha^2-2\alpha+8=0$

4 votes
2 answers
231 views

The phrase "up to"

4 votes
2 answers
287 views

Consequence of First Homomorphism Theorem?

4 votes
1 answer
44 views

How can a decomposable element be transgressive without violating Leibniz rule?

3 votes
0 answers
62 views

Where do $k$-invariants actually live?

3 votes
1 answer
620 views

Intuition for Burnside's Lemma (aka Cauchy-Frobenius Lemma)

3 votes
2 answers
201 views

Defining homotopy fiber "naturally"

3 votes
1 answer
233 views

Integrally closed non-GCD domain

3 votes
1 answer
786 views

Quotient of injective module which is not injective

3 votes
1 answer
349 views

Use of language "the exponent to which $10$ belongs $\pmod p$"

2 votes
0 answers
34 views

Do we have a nice classification of injective modules over a local ring?

2 votes
0 answers
144 views

associativity of smash product up to homotopy equivalence

2 votes
1 answer
239 views

Understanding Abel-Ruffini

2 votes
1 answer
416 views

There must be a monochromatic odd cycle in $t$-coloring of $K_{2^t+1}$