What is the namespace for the math functions in XPath and XSLT?

XPath defines some mathematical functions. These functions come with the prefix math:. I wonder what is the namespace for 'math'. It turns out that I can't execute the following XSLT in XMLSpy:

<xsl:stylesheet version="3.0"
     xmlns:xsl="http://ift.tt/tCZ8VR">
   <xsl:template match="/">
      <xsl:value-of select="math:pi()"/>
   </xsl:template>
</xsl:stylesheet>