Hello Axel, I have followed your advice : I removed the \usepackage[utf8]{inputenc} line and changed unicode into unicode-math in the \usepackage{fontspec,unicode} line. I no longer have the error message mentionning \sups being already defined, the formula is composed OK but it is displayed with what looks like Computer Modern font, instead of the expected Chalboard SE for roman characters and Euler for math symbols.
Yves -------------------------------------------------- Subscriptions, Archive, and List information, etc.: http://tug.org/mailman/listinfo/xetex