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