\newcommand{\combination}[2]{ \left( \begin{array}{c} #1 \\ #2 \end{array} \right ) }
\newcommand{\largesum}[2]{ \begin{array}{c} \scriptstyle{#2} \\ \sum \\ \scriptstyle{#1} \end{array} }
B_n=\largesum{k=0}{n} \textstyle{\frac{1}{k!}} \largesum{r=0}{k} (-1)^r \combination{m}{r} (m-r)^n =~ ... ~