Wikipedia claims that Euler was the first do derive the wave equation in 3d. In which of his writings can I find this?
$\begingroup$ Someone pointed me to Kline, pp. 518 for 2D, 521 for 3D: archive.org/details/MathematicalThoughtFromAncientToModernTimes/… $\endgroup$– Michael BächtoldOct 19, 2019 at 16:06
1$\begingroup$ De la propagation du son, read to Berlin Academy in 1759, published 1766. Lagrange considered propagation of sound and 3D wave equation at about the same time, he corresponded with Euler and published in 1760, Nouvelles recherches sur la nature et la propagation du son. $\endgroup$– ConifoldOct 20, 2019 at 7:28