Reputation
26,805
Next tag badge:
93/100 score
31/20 answers
Badges
2 39 79
Impact
~521k people reached

1d
accepted deformation-retracts into a point / contractible : what is the difference?
Feb
5
answered Each regional of a maximal planar graph is a triangle
Feb
1
answered How can we prove that the group order is an upper bound both for the number and the dimensionalities of the irreducible representations?
Jan
29
comment If all elements of a sequence $x_n$ are in a set $M$ then $\lim_{n \rightarrow \infty} x_n$ is in $M$ as well? Why?
You should wonder why you found a counter-example and are still wondering if it is true or not. Logically speaking, if you are wondering if a statement is true and you found a counter-example, then it is not true as a general statement. (It might be true if you add some conditions though! See the answer below.)
Jan
26
comment Functor category between two small categories is not small?
I see. He actually never mentions small before he says that the hom-set between two functors is not necessarily small, I just assumed he took everything small. Thanks!
Jan
26
asked Functor category between two small categories is not small?
Jan
18
comment Is the open ball path-connected in metric space
I think his notion of normed space is over the reals, in which case the open ball is a star-shaped set, which explains the "obvious" part.
Jan
14
awarded  Popular Question
Jan
9
answered What does $\textstyle y \in \Re^{100}$ mean?
Jan
9
comment Maclaurin Series of $\frac{1}{e^x -1}$
@alex.jordan : Laurent series are called Laurent series to distinguish them from MacLaurin series. So if you want to say that you compute the MacLaurin series exists, it should not be a Laurent series of a function which admits no MacLaurin series, otherwise we invented terms for no reason. But discussing them never hurts, of course!
Jan
9
comment Help finding the determinant of a 4x4 matrix?
I downvoted because this technique only works for $2 \times 2$ and $3 \times 3$ matrices (because in this case all the possible permutations of two or three elements are involved using diagonals, in the $3 \times 3$ case you need to "extend" the matrix though to allow permutations with negative sign). In the $4 \times 4$ case this method does not work in general ; there are $24$ terms involved, not $8$.
Jan
4
awarded  Nice Answer
Jan
4
answered Help finding the determinant of a 4x4 matrix?
Jan
2
comment prove that a graph with p vertices and $2+(p-1)(p-2)/2$ edges is hamiltonian
Bleh. I am very tired. Sorry! For some reason I associated $2^p$ to the number of edges of the complete graph. Don't ask why.
Jan
2
comment prove that a graph with p vertices and $2+(p-1)(p-2)/2$ edges is hamiltonian
Do you assume the graph connected?
Jan
2
revised prove that a graph with p vertices and $2+(p-1)(p-2)/2$ edges is hamiltonian
deleted 31 characters in body
Jan
2
answered When is the function $x^{1/n}$ a monotonic function?
Jan
2
revised When is the function $x^{1/n}$ a monotonic function?
added 11 characters in body
Dec
31
comment Rudin's proof of Riesz representation theorem
@Lionville : if $A \subseteq \mathbb R$ is a set and $x_0 = \inf A$ is a real number, then by definition for every $\varepsilon >0$ there exists $x \in A$ with $x_0 \le x < x_0 + \varepsilon$. Just consider the set $A$ to be the one in the definition of $\mu(E)$.
Dec
28
comment Normal subgroup of Engel group
Looks fine to me. Although your notes do not mention Engel algebra and the Wikipedia article gives a more general definition than yours of an Engel algebra (in particular it says that in the finite-dimensional case, an Engel algebra and a nilpotent algebra are the same). Why is your algebra "the" Engel algebra? Did you pick it from some exercise? I am just being curious here.