What does $(\nabla \psi)^2$ mean? This is a question about notation. I know that for a function $\psi(x,t)$ $$\nabla\psi=\left(\frac{\partial\psi}{\partial x},\frac{\partial\psi}{\partial t}\right)$$ (the gradient of $\psi$) and $$\nabla^2 \psi=\frac{\partial^2\psi}{\partial x^2}+\frac{\partial\psi^2}{\partial t^2} $$ (the laplacian of $\psi$).

By what does $(\nabla \psi)^2$  mean?

Notes 


*

*If $(\nabla \psi)^2$ is another way of representing the laplacian than why didn't they just write $\nabla^2 \psi$ or $\Delta \psi$?

*$K$ is a scalar constant.

*If context is needed see below 



which is from "Novel Methods in Soft Matter Simulations" chapter 29 pg 21  

$\nabla \psi \cdot \nabla \psi=(u_x \hat i+u_y \hat j)
\cdot(u_x\hat i+u_y\hat j)=(u_x)^2+(u_y)^2$.
