paragraph_text: <td><math> \frac{1}{\sqrt{\sigma^{2} + \tau^{2}}} {\partial f\over \partial \sigma}\boldsymbol{\hat \sigma} + \frac{1}{\sqrt{\sigma^{2} + \tau^{2}}} {\partial f \over \partial \tau}\boldsymbol{\hat \tau} + {\partial f \over \partial z}\boldsymbol{\hat z}</math>