\gdef\en{\phi_n}
\gdef\PD#1#2{\frac{\partial #1}{\partial #2}}
\gdef\DD{\PD{}{\en}}
\gdef\SSum{\sum_{i-1}^N}
\gdef\Hess{{\mathcal H}}
\gdef\Grad{\nabla}
\gdef\Trn{^{\mathsf T}}
\gdef\Vec#1{\ensuremath{\bf{#1}}}
