NikolajK
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 Apr 14 comment Are isomorphisms stable under pullbacks? One question, can we not write $f:X\to X$ as soon as we demand $f$ is an isomorphism? Apr 14 comment Is there a symbol for always less than (or just always?) "q/n will always be less or equal to n" - what do you mean by this? How does your q depend on n here? Secondly, "sometimes" is not a notion often used in a formal context, even if modal logic may capture it. Mar 27 comment On the integral of $e^{aix}$. Emma reporting in. Mar 16 awarded Nice Answer Mar 14 answered Showing that $\int\limits_{-a}^a \frac{f(x)}{1+e^{x}} \mathrm dx = \int\limits_0^a f(x) \mathrm dx$, when $f$ is even Feb 19 comment How does the Tarski axiom relate to the Grothendieck universe? @FabioLucchini: Yeah, in fact I've writen parts of the Wikipedia article on Tarskiâ€“Grothendieck set theory and its Mizar motivations. I can't believe 3 years passed. Jan 22 awarded Popular Question Jan 19 accepted Is the metric on the circle, induced from the plane, not a flat one? Jan 13 comment Is the metric on the circle, induced from the plane, not a flat one? Not sure if I follow. Whatever the value of the distance between two point, the metrics are still flat right? Jan 13 answered Let $f(x) = (x^n-1)/(x-1)$. Why does $f(1)=n$? Jan 13 revised Is the metric on the circle, induced from the plane, not a flat one? added 3 characters in body Jan 13 asked Is the metric on the circle, induced from the plane, not a flat one? Dec 29 comment What is the $\tau$ symbol in the Bourbaki text? Thank you! If $(\exists x)R$ is alias for $R[\tau_x(R)]$, then what if $(\exists x)R$ is actually false? You imply $\tau_x(R)$ still makes sense then. Dec 22 revised A question on concrete category edited body Dec 5 comment Power series representation of $\frac{1+x}{1-x}$ explanation? Remark: Note that your $\frac{1}{x}$-move spoils the possibility to evaluate your sum representation for $\frac{1+x}{1-x}$ at $x=0$. Similat story for $\lim_{x\to\infty}$. Simulatnously, the naive termwise substraction of infinite sums makes each $x^n-\frac{1}{x^n}$ well defined at $x=1$, whereas $\frac{1+x}{1-x}$ is not. Dec 5 answered A question on concrete category Dec 4 awarded Popular Question Nov 27 comment Pullbacks - question 5.7.2 from Awodey's *Category Theory* Btw. I asked a semi-related question here here. Nov 27 comment Pullbacks - question 5.7.2 from Awodey's *Category Theory* For sure you want to add to the square in the second question (with corners being $X, A, B$ and $A\times_X B$) an arrow $f:Y\to X$. What's eventually asked for is supposed to be a pullback square over $Y$. Ask yourself: What are ways in which the 3 remaining corners could be obtained from the 5 you already got? For the last part, think about what you start with (the diagram in that question) and what you're supposed to use (the diagram in the first question). Use the result of the second to bridge the two. Nov 27 comment Let $f_n(x) =\frac{x}{1+nx^2}$ and what function does this sequence converge to? How come you asked 50 questions on Math SE but don't know the basics of formatting here?