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 Feb 15 asked If the covariant derivative vanishes Jan 27 asked Integration over two sub manifolds Jan 17 accepted Which initial functions can be solved by separation of variables Jan 16 comment Which initial functions can be solved by separation of variables Thank you very much. Your last formula is very elegant and it seems as if the function $u(x,t)$ is not differentiable in general, even not for $f,h\in C^2(0,1)$. When we assume further that $f,h$ can be extendet periodically in a $C^2$ fashion then the right hand side would be $C^2$ as well. Is this the most general assumption to guarantee the existence of a $C^2$ solution? Jan 13 revised Which initial functions can be solved by separation of variables added 630 characters in body; edited tags Jan 12 asked Which initial functions can be solved by separation of variables Jan 12 comment convergence of series in inner product space Perfect answer! Thank you very much! Jan 12 accepted convergence of series in inner product space Jan 12 revised convergence of series in inner product space added 9 characters in body Jan 12 asked convergence of series in inner product space Jan 12 accepted dihedral group and its generators Dec 21 asked dihedral group and its generators Dec 18 revised Group actions by semi-direct products of groups deleted 295 characters in body Dec 18 revised Group actions by semi-direct products of groups added 217 characters in body Dec 17 revised Group actions by semi-direct products of groups deleted 92 characters in body Dec 17 comment Group actions by semi-direct products of groups I have another question: Is there some rule of thumb how to recognize which semi-direct product the author had in mind? For instance is it common to assume, or to interpret the situation as an inner semi-direct product whenever there is no group action mentioned? Dec 17 comment Group actions by semi-direct products of groups Thank you very much for your help Alex G. I did not even thought of this possibility. I suppose the bigger group is $K:=\lbrace x\mapsto A(x+z): A\in G, z\in\mathbb{Z}^2 \rbrace$. ( One can show $\mathbb{Z}^2$ is a normal subgroup of K). Are my calculations correct so far? But I could not figure out why $\mathbb{R}^2/K$ equals a square with side length 1/2 Dec 17 asked Group actions by semi-direct products of groups Dec 16 comment Group of rotations of rational multiples of $\pi$ Thank you for your comment. Your first sentence does not seem to be correct. Consider the example $\frac{2}{3}\pi$ and $\frac{\pi}{3}$. The fractions are reduced and obviously the denominators are equal. But the groups are different. Am I missing something? Dec 16 asked Group of rotations of rational multiples of $\pi$