\documentclass[10pt,a4paper]{article}
\usepackage[latin1]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{mathbrack}
\usepackage{amssymb}
\begin{document}
$ \overbrack{x+\cdots+x}^{k\rm\;times} \qquad
\underbrack{x+y+z}_{>\,0} \qquad
\overparen{x+\cdots+x}^{k\rm\;times} \qquad
\underparen{x+y+z}_{>\,0} $
\end{document}