\[M_{1}=\begin{pmatrix}n\\
\overbrace{1,\ldots,1}^{a_{1}},\ldots,\overbrace{n,\ldots,n}^{a_{n}}\end{%
pmatrix}=\frac{n!}{(1!)^{a_{1}}(2!)^{a_{2}}\cdots(n!)^{a_{n}}}.\]