OpenMath is an extensible standard for representing the semantics of mathematical objects
OpenMath homepage

Om -- a general public discussion list for any aspects of OpenMath
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Advanced Language Technologies for Mathematical Markup, research presentation (PDF).

OpenMath Content Dictionary Wiki

Advanced Language Technologies for Mathematical Markup, slide show.

Easily Editing and Browsing Complex OpenMath Markup with SWiM, slide show.

Mathematical Semantic Markup in a Wiki, slide show (PDF)