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Dec
12
answered $f$ continuous on $(a,b)$ and $|f|$ differentiable on $(a,b)$; is $f$ differentiable in $(a,b)$?
Dec
12
comment $f$ continuous on $(a,b)$ and $|f|$ differentiable on $(a,b)$; is $f$ differentiable in $(a,b)$?
The function does not necessarily change its sign every time $f(x)=0$ (consider $f(x)=x^2$). Of course only those where it does may be "problematic"; it's just the "i.e." that is wrong.
Dec
9
awarded  Notable Question
Nov
1
answered Show the following is a group $a * b=a+b+ab$
Nov
1
answered Determine the complex function $f\left ( z \right )$
Nov
1
revised measurable functions. Why defined like this?
edited body
Nov
1
answered measurable functions. Why defined like this?
Oct
31
revised What mathematical object accepts a sequence as input?
added 113 characters in body
Oct
31
answered What mathematical object accepts a sequence as input?
Oct
31
comment Is there a more concise way to indicate many elements that belong to a set?
@MartinBalog: (1) No, if $(a,b,\ldots,l)\in A\times B\times\ldots\times L$ then $a\in A$, $b\in B$, …, $l\in L$.
Oct
31
comment My proof is correct ? (Topology)
What is $\mathcal V_x$?
Oct
31
answered boolean equation to truth table
Oct
31
comment $\mathbb{R}$ is a countable union of closed sets, then at least one of them has nonempty interior
In the "related" section, this turns up: math.stackexchange.com/questions/88632/… — I guess that should be helpful.
Oct
31
revised boolean equation to truth table
MathJax, typos
Oct
31
comment boolean equation to truth table
Ah, so the "output of the truth table" is the column giving the truth values of the expression? In that case, how did you sort the rows? Is it "FFF,FFT,FTF,…" or "FFF,TFF,FTF,…"?
Oct
31
comment boolean equation to truth table
What is the output of a truth table?
Oct
29
comment Is zero a prime number?
Why do you refer to zero as "he"?
Oct
29
comment Where, if ever, does the decimal representation of $\pi$ repeat its initial segment?
@MarcPaul: Indeed, a number can even infinitely often repeat itself without being rational. Consider $0.10100101000101001010000101001010001010010100000\ldots$. It repeats itself after the second digit ($0.10\color{red}{10}\ldots$), after the fifth digit ($0.10100\color{red}{10100}\ldots$), after the 11th digit ($10100101000\color{red}{10100101000}\ldots$), …
Oct
28
answered convergent sequence / prove of reordering rule
Oct
28
revised convergent sequence / prove of reordering rule
added 3 characters in body