User:Milton Beychok/Sandbox

${\bigg (}{\frac {\;{\boldsymbol {\partial }}{\boldsymbol {\varphi }}}{{\boldsymbol {\partial }}{\boldsymbol {t}}}}{\bigg )}_{\boldsymbol {x,y,z}}$

where ${\boldsymbol {\varphi }}$ represents the dependent variable and ${\boldsymbol {t}}$ represents time.