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				% 9-12-1.tex
				\documentclass{article}
				\usepackage{amsmath}
				\newcommand{\dif}{\mathrm{d}}
				\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}