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Apr
8
accepted Why are functors exact if they preserve all short exact sequences?
Apr
8
accepted The pushout of an open/closed injective map is open/closed
Mar
15
accepted Can the cohomology ring of the two-fold torus be calculated abstractly?
Feb
1
accepted Extending homotopies to contract an ascending union of contractible spaces
Jan
31
accepted Which maps on the zeroth homology do actually come from continuous maps?
Jan
29
accepted Is reduced homology a full functor on connected spaces?
Jan
27
accepted How is induction justified in intuitionistic logic?
Dec
29
accepted Equivalent characterizations of group objects
Dec
14
accepted Are the polynomial functions on $S^1$ dense in $C(S^1,ℂ)$?
Dec
14
accepted Can $ℂ$ be viewed as a (nontrivial) field of fractions?
Nov
29
accepted Why is every category in which all (even large) limits and colimits exist thin?
Nov
6
accepted Is a set open in a product of spaces if all its segments are open in their factors?
Oct
27
accepted Does $f ≥ g$ always imply $f^{-1} ≤ g^{-1}$ for invertible $f, g$?
Sep
23
accepted Example of a convex set whose closure is not convex?
May
20
accepted Embeddings $A → B → A$, but $A \not\cong B$?
Apr
10
accepted How could I have found the closed form of $\sum_{k=1}^n \frac{k}{(k+1)!}$ in advance?
Mar
18
accepted Is every left maximal ideal the annihilator of a simple left module?
May
28
accepted Taylor expansion of $(1+x)^α$ to binomial series – why does the remainder term converge?
May
1
accepted Why is $\sum_{d \mid N} N/d ϕ(N/d) ϕ(d) = N^2 \prod_{p \mid N} (1 - 1/p^2)$?
Apr
28
accepted sine, cosine and the exponential by their differential equations