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Jeremy Rickard

Bristol, United Kingdom

Algebraist at the University of Bristol.

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Top Questions
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75
$A$ is isomorphic to $A \oplus \mathbb{Z}^2$, but not to $A \oplus \mathbb{Z}$
mathoverflow.net

57
Are there any nontrivial abelian categories with only finitely many objects?
mathoverflow.net

49
Example of an unnatural isomorphism
mathoverflow.net

46
Is it consistent with ZF that $V \to V^{\ast \ast}$ is always an isomorphism?
mathoverflow.net

45
Do vector spaces without choice satisfy Cantor-Schroeder-Bernstein?
mathoverflow.net

35
Does the symmetric group on an infinite set have a minimal generating set?
mathoverflow.net

30
Can a ring without a unit element have a subring with a unit element?
math.stackexchange.com

30
Are there only finitely many associative algebras of fixed dimension?
mathoverflow.net

29
If $A$, $B$ are abelian groups such that $\mathrm{Hom}(A, G) \cong \mathrm{Hom}(B, G)$ for all abelian groups $G$, must $A$ and $B$ be isomorphic?
mathoverflow.net

27
Cocomplete but not complete abelian category
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27
A question about representations of finite groups
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26
How much of a group $G$ is determined by the category of $G$-sets?
math.stackexchange.com

25
Additive category that is not abelian
math.stackexchange.com

25
Combinatorics Problem: $\sum _{k=0}^{s-1} \binom{n}{k}=\sum _{k=1}^s 2^{k-1} \binom{n-k}{s-k}$
mathoverflow.net

24
Non-isomorphic graphs with bijective graph homomorphisms in both directions between them
mathoverflow.net

24
Is it possible that a left coset of $H$ contains more than one right coset of $H$?
math.stackexchange.com

23
Application of Hilbert's basis theorem in representation theory
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23
Is deciding if one planar graph is dual to another really NP-hard (Wikipedia claim)?
mathoverflow.net

22
Example of an abelian category with enough projectives and injectives which are not dual
mathoverflow.net

21
Do mutually dual finite vector spaces have the same orbit cardinalities under a linear group action?
mathoverflow.net

20
Is the homotopy category of an abelian model category abelian?
mathoverflow.net

20
Classification of subgroups of finitely generated abelian groups
mathoverflow.net

20
Is the functor from the unbounded derived category of coherent sheaves into the derived category of quasi-coherent sheaves fully faithful?
mathoverflow.net

18
Are all vector-space valued functors on sets free?
mathoverflow.net

18
Recovering an abelian category from the Ext of its simple objects
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18
If the tensor power $M^{\otimes n} = 0$, is it possible that $M^{\otimes n-1}$ is nonzero?
math.stackexchange.com

18
Is it true that a flat module is torsion-free over an arbitrary ring? Does the reverse implication hold for finitely generated modules?
math.stackexchange.com

17
Torsion-free abelian group $A$ such that $A \not \simeq A \oplus \Bbb Z \simeq A \oplus \Bbb Z^2$
mathoverflow.net

17
Tilting Objects in BGG Categories $\mathcal{O}$
mathoverflow.net

17
A group whose automorphism group is cyclic
mathoverflow.net

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