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 Oct21 revised Euler-Lagrange equation problem added 132 characters in body Oct21 answered Euler-Lagrange equation problem Oct17 revised Finding $\lim_{n \to \infty} \frac{\sqrt{n!}}{2^n}$ changed in title $x$ into $n$ Oct17 suggested approved edit on Finding $\lim_{n \to \infty} \frac{\sqrt{n!}}{2^n}$ Oct17 comment Topological structure of a quotient of ${\rm{SU}}(2)\times{\rm{SU}}(2)$ Not an answer, but synonyms: $\mathbb{S}^3\times \mathbb{S}^3/\mathbb{Z}_2$ is the double cover, Spin$(4)$, of SO(4). Oct16 comment Christoffel symbols in Differential geometry iff proof But Christoffel symbols are objects (non-tensors) with three indices (unless you contract two of them). I.e. $$\Gamma^\alpha_{\beta\gamma}=\frac{1}{2}g^{\alpha\mu}(g_{\mu \beta,\gamma}+g_{\mu \gamma,\beta}-g_{\beta\gamma,\mu}).$$ Oct14 suggested rejected edit on How to show that $\frac{1}{\tan(x/2)}=2 \sum_{j=1}^{\infty}\sin(jx)$ in Cesàro way/sense? Oct14 comment Christoffel symbols in Differential geometry iff proof Hi! I'd like to know what does $H$ mean? where are the Christoffel symbols here? Oct14 comment $4 \times 4$ matrix and its inverse. Is my method ok? I'm sure there are more modern references, but in Jackson's book on Classical Electrodynamics (chapters 11 and 12) you can find a complete physical approach (I don't think special relativity should be read as math, for it is definitely not math). By definition, $\gamma$ satisfies $\gamma^2-\beta^2\gamma^2=1$, so you can set $\phi=\cosh^{-1}\gamma$ with a sign ambiguity to be fixed by $\sinh \phi=\beta\gamma$. Then $\gamma^2-\beta^2\gamma^2=1=\cosh^2(\phi)-\sinh^2(\phi)$. Oct12 awarded Promoter Oct12 answered $4 \times 4$ matrix and its inverse. Is my method ok? Sep17 awarded Autobiographer Aug11 awarded Teacher Jun24 asked Dixmier-Douady class: computations Jun23 comment Compact resolvent VS certain boundedness condition I see. However, one doesn't have control on the boundedness of the elements in $A$. Anyway, very helpful answer. Jun23 accepted Compact resolvent VS certain boundedness condition Jun22 revised Compact resolvent VS certain boundedness condition deleted 43 characters in body Jun22 revised Compact resolvent VS certain boundedness condition Changed condition 2. from "$(D+\lambda)^{-1}$ is a compact operator for each $\lambda\notin i\mathbb{R}$" Jun21 awarded Commentator Jun21 revised Compact resolvent VS certain boundedness condition added 2 characters in body