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Martin Brandenburg's user avatar
Martin Brandenburg's user avatar
Martin Brandenburg
  • Member for 15 years, 4 months
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  • Berlin, Germany
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297
Examples of common false beliefs in mathematics
May 5, 2010
116
A proof of $\dim(R[T])=\dim(R)+1$ without prime ideals?
Jun 21, 2014
96
$A$ is isomorphic to $A \oplus \mathbb{Z}^2$, but not to $A \oplus \mathbb{Z}$
Sep 12, 2015
88
When is $A$ isomorphic to $A^3$?
Dec 30, 2009
78
Rigidity of the category of schemes
Feb 28, 2011
74
$R$ is isomorphic to $R[X,Y]$, but not to $R[X]$
Dec 11, 2015
73
What are the reasons for considering rings without identity?
Apr 26, 2010
72
Results from abstract algebra which look wrong (but are true)
Jan 19, 2023
70
explicit big linearly independent sets
May 1, 2010
69
Example of an unnatural isomorphism
Aug 14, 2013
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A proof of $\dim(R[T])=\dim(R)+1$ without prime ideals?
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Nice examples of groups which are not obviously groups
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If $AB = I$ then $BA = I$
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Ring structure on the absolute Galois group of a finite field
92
Order of a product of subgroups. Prove that $o(HK) = \frac{o(H)o(K)}{o(H \cap K)}$.