Let $R$ be a topological ring (commutative with $1$) and let $R((t))$ the ring of Laurent power series. So, it is the ring containing the formal power series: $$\sum_{i\ge m} a_i t^i,\quad m\in\mathbb Z,\;a\in R$$ It is easy to see that $R((t))$ can be obtained as an ind-pro objects in the following way: $$R((t))=\varinjlim_{m\in\mathbb Z}\left […]

Let $f\colon X \rightarrow Y$ be a continuous map of topological spaces. Let $\mathcal{F}$ be a sheaf of abelian groups on $Y$. The inverse image sheaf $f^{-1}(\mathcal{F})$ is the sheaf associated to the presheaf which assigns $\operatorname{colim}_{f(U) \subset V} \mathcal{F}(V)$ for every open subset $U$ of $X$, where $V$ runs through every open subset $V$ […]

I was wondering if the following two meanings of pullback are related and how: In terms of Precomposition with a function: a function $f$ of a variable $y$, where $y$ itself is a function of another variable $x$, may be written as a function of $x$. Then $f(y(x)) \equiv g(x)$ is the pullback of $f$ […]

In the category of sets, limits and colimits can be concrete described respectively as subobjects of products and quotients of coproducts. It seems like these descriptions make sense in any elementary topos. I was wondering whether they actually describe the corresponding limits and colimits, and how to see this. Clarification. I’m asking about concrete element-based […]

Is there any connection between category-theoretic term ‘limit’ (=universal cone) over diagram, and topological term ‘limit point’ of a sequence, function, net…? To be more precise, is there a category-theoretic setting of some non-trivial topological space such that these different concepts of term ‘limit’ somehow relate? This question came to me after I saw ( […]

I am a set theorist in my orientation, and while I did take a few courses that brushed upon categorical and algebraic constructions, one has always eluded me. The inverse limit. I tried to ask one of the guys in my office, and despite a very shady explanation he ended up muttering that “you usually […]

I found at the beginning of tom Dieck’s Book the following (non proved) result Suppose $X$ is the colimit of the sequence $$ X_1 \subset X_2 \subset X_3 \subset \cdots $$ Suppose points in $X_i$ are closed. Then each compact subset $K$ of $X$ is contained in some $X_k$ Now I really don’t know how […]

I have several related questions: Do there exist colimits in the category of schemes? If not, do there exist just direct limits? Do there exist limits? If not, do there exist just inverse limits? With more generality and summarizing, with which generality there exist limits and colimits in Schemes? Then, if I have a colimit […]

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