Chris White
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 Jul 3 comment What's the difference between wave equation in PDE form and wave equation in normal form? People are often interested in sinusoidal solutions, but there are others. In general a solution is something that keeps its shape while moving at velocity $v$. And the wave equation describes many, many phenomena in physics. Jul 3 answered What's the difference between wave equation in PDE form and wave equation in normal form? Feb 8 comment Geometric meaning of $\nabla_{[i}(x^i \nabla_{j]}x^j)$ and $(\nabla_{[i}x^i )\nabla_{j]}x^j$ The determinant is just a coincidence. In general, if $A$ is a matrix with components $\nabla_i x^j$, then what you have is $(\operatorname{Tr}A)^2-\operatorname{Tr}(A^2)$. Apart from a factor of $2$, this is the sum of all products of 2 elements on the diagonal of $A$, minus the sum of all products of pairs of elements that are in reflected positions across the diagonal ($A_{12}A_{21} + A_{13}A_{31} + \cdots$). I'm not sure this is particularly useful though, or that it has a physical interpretation. Dec 10 awarded Caucus Oct 2 awarded Yearling Sep 30 awarded Commentator Sep 30 comment Very *mathematical* general physics book I'm just saying the OP may not know what they're asking for. Someone who's being taught out of that book is not in a position to understand any of the physics in the books you've mentioned, and probably very little of the math. Sep 30 comment Very *mathematical* general physics book You should know, since it appears you've not read it, that Resnick et al. is a middle school level heuristic survey of random topics in physics used to write GRE questions and nothing else. Physics for poets, basically. One need not go so advanced as the things you've listed in order to get something "mathematical" compared to it. Sep 24 awarded Autobiographer Sep 16 comment Notation regarding the continuity equation for conservation of mass Mathjax supports \iint and \oint but not \oiint. In a pinch there's \unicode{x222F} for $\unicode{x222F}$, but it doesn't scale beyond inline math. Aug 9 awarded Citizen Patrol Mar 3 awarded Student Mar 3 asked What is the generalization of Gauss's Theorem to a manifold? Dec 22 awarded Enlightened Dec 22 awarded Nice Answer Nov 1 answered How can a single integral equal a triple integral? Oct 29 awarded Quorum Oct 2 awarded Yearling Aug 26 comment Solving ODE with negative expansion power series @dingo_d Well, Math Overflow is generally only for research-level math. If your question is just needing an application of well-known ODE theory, it belongs on Math Stackexchange. If it calls upon physical principles in some meaningful way (beyond just "this came up while doing physics"), then it is okay here. But I doubt it will be particularly well-received on Math Overflow. In general, though, if you feel the question should be moved, you can ping a moderator and request they migrate it for you, to avoid cross-site duplication. Jun 28 comment Analytical Solution for Elastic Bar under applied end velocity Existence of solutions sounds more like a math topic, unless you want to know if the setup is physically reasonable enough to admit a solution. I suggest migrating to mathematics, though in that case the users there should take into account that the word "analytical" here probably means "in terms of common functions" rather than simply "converges to its Taylor series."