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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