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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 Tarski–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?