Anti-periodic potentials and Bloch theorem

 $$V(x+a)=-V(x)$$

$$|\psi(x+a)|^2=-|\psi(x)|^2+1?$$

Viscosity in a Plasma

 $$\boldsymbol{\tau}=\mu(\nabla\textbf{A}+\nabla\textbf{A}^T)$$ where \(\textbf{A}\) is the magnetic vector potential.

Mathematical possibilities of Newton's Third Law

 $$\textbf{F}(\textbf{r}_1,\textbf{r}_2)=\textbf{F}(\textbf{r}_2,\textbf{r}_1)$$

The simplest possibilities are 

$$r=|\textbf{r}_1-\textbf{r}_2|$$$$\textbf{r}=\textbf{r}_1+\textbf{r}_2$$