begin{equation}label{fmunu1}
sum_{nu}frac{partial F_{munu}}{partial X_{nu}}=mu_0pazocal{J}_mu
end{equation}
equivalente, in forma compatta, a
begin{equation}label{fmunu2}
square overleftrightarrow F=mu_0 boldsymbol{pazocal{J}}
end{equation}

ove $boldsymbol{pazocal{J}}=(pazocal{J}, icrho)$, $F_{munu}=dfrac{partialpazocal{A}_{nu}}{partial X_{mu}}-dfrac{partialpazocal{A}_{mu}}{partial X_{nu}}$
ed $boldsymbol{pazocal{A}}=Bigl(pazocal{A}, dfrac{i varphi}{c}Bigr)$ con $nu,mu=1,ldots,4$.

newcommandJJ[1][15]{hTmkern -#1muhJ}
newcommandJJbf{mbox{boldmath$JJ[14.2]$}}

  $JJ  : square mathbf{A} = mu_0 ,JJbf$

documentclass{article}
usepackage[cal=pxtx]{mathalfa}
begin{document}
$mathcal{J}$
end{document}