% 9-12-1.tex
\documentclass{article}
\usepackage{amsmath}
\newcommand{\dif}{\mathrm3zptp35}
\begin{document}
\begin{equation}
\sum_{\substack{k_0,k_1,\ldots>0\\
k_0+k_1+\cdots=n}} F(k_i)
\end{equation}

\begin{equation*}
\sum_{\begin{subarray}{l}
k_0,k_1,\ldots>0\\k_0+k_1+\cdots=n
\end{subarray}}  F(k_i)
\end{equation*}

\begin{gather*}
\iint\limits_D f(x,y)\,\dif x\dif y\qquad
\iiint\limits_V f(x,y,z)\,\dif V \\
\iiiint\limits_G f(x,y,z,t)\,\dif G\qquad
\idotsint\limits_U f(x_1,\dots,x_n)\,\dif U
\end{gather*}

\[
\begin{pmatrix} a_{11} & a_{12} & \dots & a_{1n}\\
a_{21} & a_{22} & \dots & a_{2n}\\
\hdotsfor{4}\\
a_{n1} & a_{n2} & \dots & a_{nn}
\end{pmatrix}
\]
\end{document}