André
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 17h accepted Possible angles between roots in a root system 18h asked Possible angles between roots in a root system May24 comment Rootspace decomposition of a Lie algebra I am reading Humphreys, too but I do not immediately see why the sum is direct. May23 accepted Stone-Čech compactification of discrete space May23 asked Rootspace decomposition of a Lie algebra Apr21 comment Should an object in the category always be a formal mathematical structure? Maybe you shoud first try to read articles over ZFC, such as here en.wikipedia.org/wiki/Zermelo%E2%80%93Fraenkel_set_theory. Furthermore you can't define something to be large. For example the category of sets and functions between them is not small since the objects do not form a set. (There can't be the set of all sets.) So the objects form a so called class, and hence the category of sets is large. Apr21 comment Should an object in the category always be a formal mathematical structure? Maybe you are looking for the difference between small and so called large categories. In small categories the objects form really a set and also the morphisms between objects form sets. Besides that you can always define categories which are not related to any popular mathematical structure, of course. Apr16 comment $2D$ plane geometry inequality Then, indeed all points above the line you drawed. (including the line itself) Apr16 answered $2D$ plane geometry inequality Apr16 comment Is a subspace of functions that essentially depend only on one variable closed? I guess he means the functions $f$ which are equal, when hold constant in the second variable. Apr16 revised Graph Theory Matchings latex added Apr16 suggested approved edit on Graph Theory Matchings Apr16 accepted Finite union of compact sets is compact Apr13 comment What is the reverse image of $f:[0,6\pi] \rightarrow \mathbb{R}$, $f = sinx$? The reverse image is a group ? Apr10 comment Is a closed convex set $E$ in $\mathbb{R}^n$ equal to the closure of its interior? I just added the link. Sorry. Apr10 comment Is a closed convex set $E$ in $\mathbb{R}^n$ equal to the closure of its interior? There is a theorem wich states that a compact convex set is the convex hull of its extreme points. See en.wikipedia.org/wiki/Krein%E2%80%93Milman_theorem Apr10 answered Prove that if $m$ and $n$ are integers and $mn$ is even, then $m$ is even or $n$ is even. Apr9 comment Existence of bijection that reorders elements? Edit is ok, I guess. Then just ignore my answer :) Apr9 answered Existence of bijection that reorders elements? Apr9 answered why union and Cartesian product of infinitely many compact sets is not compact