1,532 reputation
619
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location Berlin, Germany
age 26
visits member for 3 years, 9 months
seen 7 hours ago

I got my Master degree in Pure Mathematics in Rome, and now I'm a PhD-student in Berlin.

My main research interest is Arithmetic Geometry, but I'm here mainly because I like to keep an open eye towards a variety of fields in Mathematics!


Feb
6
comment some question of sheaf
The assumption $X=U$ is harmless. Indeed if $U,V$ are two open subsets containing $Z$ then $\Gamma_{Z\cap V}(V,\mathcal{F}|_V)\simeq\Gamma_Z(X,\mathcal{F})\simeq \Gamma_{U\cap Z}(U,\mathcal{F}|_U)$. Regarding the fact that $Z$ is only locally closed in $X$ I have been indeed a bit sloppy. As it is stated my proof works assuming $Z$ closed, it works also for $Z$ locally closed but you have to be a bit more careful when applying arguments involving open coverings of $X$. When I'll have some time I'll write down details about it. Thank you for pointing it out!
Feb
6
answered some question of sheaf
Jan
23
comment $z \sin z \sin 1/z$ does not have singularities, why?
Ooops! Despite the use of the variable, the words "pole" and "residue" and the tag I was looking at the real situation... I'm sorry!
Jan
23
comment $z \sin z \sin 1/z$ does not have singularities, why?
Try to compute the limit! $\sin(\frac{1}{z})$ is not defined at $z=0$, but it is certainly bounded in a neighborhood. While $z$ and $\sin(z)$...
Jan
15
comment The lower bound on the number of numbers needed to fill a matrix in a special way.
Done! I'm sorry for your original problem, but I would say this is quite common in Mathematics...
Jan
15
answered The lower bound on the number of numbers needed to fill a matrix in a special way.
Jan
14
comment The lower bound on the number of numbers needed to fill a matrix in a special way.
Absolutely correct! You can think about writing it down as an answer and then accept it, so people will not consider this question as open anymore!
Jan
14
comment Prove a function is primitive recursive
Perfect! ;) For your future participation in Math.SE remember that people do care about how many answers did you accept, so keep going!
Jan
14
comment The lower bound on the number of numbers needed to fill a matrix in a special way.
No problems, but it is wrong as well. Consider $n=3$, then $k=2$. Let $A=\{a,b,c\}$, and consider the $2\times 2$-matrix with $a$ in both the places in the diagonal and $b$ otherwise. This respect your condition but you cannot find $c$ in it. Perhaps we can try to find the smallest $k(n)$ such that your condition on rows and columns implies that you can find all the elements of $A$... ...but the answer would be $k(n)=n$ always, can you prove it?
Jan
14
comment The lower bound on the number of numbers needed to fill a matrix in a special way.
This is wrong. Pick $n=2$, then...
Jan
14
comment Prove a function is primitive recursive
You are welcome! The problem with your solution seems to me that $f(x-a)$ could be not well defined. In the sense that a primitive recursive function goes from $\mathbb{N}$ to $\mathbb{N}$ and if $x<a$ then $x-a\notin\mathbb{N}$. Moreover you should observe that the composition of primitive recursive functions is primitive recursive. If you want to be sure to have understood a good exercise could be to write $f_n(x)=x+n$ only in terms of projective and successor functions. Lastly, do you know how to accept an answer?
Jan
14
answered Prove a function is primitive recursive
Jan
14
comment Prove a function is primitive recursive
Welcome to Math.SE! You could have considered the addition of this link to your question: en.wikipedia.org/wiki/Primitive_recursive_function . In specific the paragraph "Addition" in the page above should answer your question. Is it correct?
Jan
12
accepted Computation of a (probably) tricky limit.
Jan
10
comment Computation of a (probably) tricky limit.
@Marek Thank you very much! I unzipped your comment and you`re totally right (there is still a factor 2 missing, but I'm confident I'll find it once reviewing the computation)! If you want to put it as an answer I will accept and upvote it as soon as possible! :)
Jan
10
revised Computation of a (probably) tricky limit.
edited body
Jan
10
comment Evaluate a double infinite summation
Where does this problem come from? Are you familiar with the notion of Eisenstein series? en.wikipedia.org/wiki/Eisenstein_series It seems quite close to it. But I don't see straight away any explicit relation between the two. If you could provide some more details we could help you better.
Jan
10
comment Evaluate a double infinite summation
What is your question?
Jan
10
comment What is $\operatorname{Ext}_{\mathbb{Z}} (\mathbb{Z}/m\mathbb{Z},\mathbb{Z}/n\mathbb{Z})$?
Are you looking at $\mathbb{Z}/m\mathbb{Z}$ and $\mathbb{Z}/n\mathbb{Z}$ as $\mathbb{Z}$-modules?
Jan
10
revised Computation of a (probably) tricky limit.
added 10 characters in body