If MathML is being used by an application that conforms to the Namespaces in XML Recommendation, then the following namespace should be used:
Typical usage would be:
<math xmlns="http://www.w3.org/1998/Math/MathML"> . . .
It is not a requirement of the XML Namespaces Recommendation that there is any accessible resource at the URI used for a namespace name, although for information purposes, a copy of this file may be downloaded from the above URL.
The current MathML Recommendation may be obtained from http://www.w3.org/TR/MathML
At the present time (July 2011) the current version is MathML 3.0, links to this and earlier versions are listed below.