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This is a basic version of text.
\documentclass[a4paper,oneside,titlepage,12pt]{article}

\usepackage[cm-default,no-math]{fontspec}
\usepackage{xunicode}
\usepackage{xltxtra}
\usepackage{amsmath}
\usepackage{xgreek}

\setmainfont[Mapping=tex-text]{DejaVu Sans}
\parindent=0in

\begin{document}

$A ...

I have a similar problem with XeLaTeX

\documentclass[a4paper,oneside,titlepage,12pt]{article}

\usepackage[cm-default]{fontspec}
\usepackage{xunicode}
\usepackage{xltxtra}
\usepackage{amsmath}
\usepackage{amssymb}

\usepackage{xgreek}

\setmainfont[Mapping=tex-text]{DejaVu Sans}

In document ...