Christian Blatter
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 Apr7 comment $\varepsilon$-$\delta$ proof of continuity of floor function $\lfloor x\rfloor$ How could you even think that the floor function is everywhere continuous? Apr7 comment Arc subtended by inner infinitesimal angle My first suggestion is that you draw a comprehensible figure. Apr7 comment Verifying proof that $f$ has an inflection point at zero if $f$ is a function that satisfies a given set of hypotheses. Your argument is correct. What is not so correct, and makes your question difficult to understand, is the presentation of the hypotheses for $f$. Apr6 comment Find all Real polynomials. Missing is an argument of the following kind: When $f_1$ and $f_2$ are two solutions of the problem then $f_1-f_2$ has to be zero at $x\in\{2,3,5\}$. Therefore $f_1(x)-f_2(x)=(x-2)(x-3)(x-5) g(x)$ for some polynomial $g$. Apr6 comment the relationship between $f^{-1}(x)$ and $x$ Use another letter, as $y$, for numbers in the range of $f$, i.e., in the domain of $f^{-1}$. Note that the subset $f^{-1}(\{y\})\subset\ ]0,1[\$ is defined even if $f$ is not injective, and $x>y$ holds for all $x$ in this subset. Apr6 comment Cosine of an area Now you have it right! Apr5 answered Calculate $\sqrt{5}$ using taylor series to the accuracy of 3 digit after the point $(0.5*10^{-3})$. Apr5 comment Has lack of mathematical rigour killed anybody before? This is not about "mathematical rigour" but about wrong thinking. Apr5 comment Cosine of an area What "can be easily proved" is wrong: The scaling factor for segments depends on their direction. Segments $\sigma\subset B$ orthogonal to $g:=A\wedge B$ are scaled by a factor $\cos\theta$, and segments parallel to $g$ keep their length under projection. Now find out how the area of suitable rectangles is scaled. It is a nontrivial theorem that all areas are scaled by the same factor. Apr5 answered Trick with differentials from $\frac{dr}{ds} \to \frac{dr}{dt}$ Apr5 comment What does the sign “$=$” exact meanings? Thank you for the quote! Apr5 comment Identical objects into identical boxes This question is a duplicate. There is an accepted answer here: math.stackexchange.com/questions/1219692/… Apr5 comment My try at the completeness axiom proof It's unclear what you assume about ${\mathbb R}$, and what you want to prove. Apr5 answered Proving positivity of the exponential function Apr5 comment Proving positivity of the exponential function Put $x:=-20$ in your second displayed formula! Apr5 comment Having difficulty understanding topological groups. Nondiscrete topologies on finite sets are a nightmare. Basic examples of topological groups are $({\mathbb R},+)$, ${\mathbb R}_{>0},\times)$, $S^1$, $GL(n,{\mathbb R})$. Apr4 revised $12$ Identical balls can be placed into $3$ identical boxes, added 23 characters in body Apr4 revised If $\lim_{x\to0} f(x)+g(x)$ and $\lim_{x\to0} f(x)g(x)$ exist simultaneously, are there any $f(x)$ and $g(x)$ that do not have limit added 62 characters in body Apr4 answered Showing that the function $f(x,y)=x\sin y+y\cos x$ is Lipschitz Apr4 revised Jacobian matrix dimension problem? added 515 characters in body