The code is here. Also How can i add underbraces from fourier?
\documentclass[11pt,a4paper]{article}
\usepackage[english]{babel}
\usepackage[utf8x]{inputenc}
\usepackage{ucs}
\usepackage{amsmath}
\usepackage{amsfonts,theorem}
\usepackage{amssymb}
\usepackage{newtxtext}
\usepackage{newtxmath}
\DeclareSymbolFont{mdsymbols} {OMS}{mdput}{m}{n}
\DeclareSymbolFont{mdlargesymbols}{OMX}{mdput}{m}{n}
\DeclareMathDelimiter{\lbrace}
{\mathopen}{mdsymbols}{"66}{mdlargesymbols}{"08}
\DeclareMathDelimiter{\rbrace}
{\mathclose}{mdsymbols}{"67}{mdlargesymbols}{"09}
\renewcommand{\textbraceleft}{%
{\fontfamily{mdput}\fontencoding{OMS}\selectfont\char"66}}
\renewcommand{\textbraceright}{%
{\fontfamily{mdput}\fontencoding{OMS}\selectfont\char"67}}
\let\sum\relax
\DeclareMathSymbol{\sum}{\mathop}{mdlargesymbols}{"50}
%\let\sum\relax
%\DeclareMathSymbol{\sum}{\mathop}{mdlargesymbols}{"50} % i want this from computer modern.
\begin{document}
\[\sum\]
should come from computer modern.
\[ \left\{
\begin{array}
[c]{ccc}%
x & , & 0<x<1\\
\sin x & , & x\geqslant1\\
x^2 & , & x<0\\
\end{array}
\right. \]
braces from fourier.
\end{document}
\sum
.newtxmath
, why should you use a visually incompatible symbol from Computer Modern?