Communities (4)
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- 62 $A$ is isomorphic to $A \oplus \mathbb{Z}^2$, but not to $A \oplus \mathbb{Z}$
- 50 Are there any nontrivial abelian categories with only finitely many objects?
- 44 Example of an unnatural isomorphism
- 41 Is it consistent with ZF that $V \to V^{\ast \ast}$ is always an isomorphism?
- 35 Does the symmetric group on an infinite set have a minimal generating set?
- 33 Word evaluating to a group element and its inverse with different frequency
- 28 Can a ring without a unit element have a subring with a unit element?
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