Upon my research of polarization of light I stumbled upon the following formula: ![enter image description here][1] What do the three dots mean? I have never seen them before. Edit: Found this notation in the book "Nonlinear fiber optics", $P_{NL}$ is the nonlinear polarization component, while $\chi$ is unknown to me (haven't found it, but I assume that it is a tensor for the polarization) [1]: https://i.sstatic.net/tghxo.png