XSLT Cookbook

Java Extension Function Using the Class Format Namespace

<xsl:stylesheet 
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:xalan="http://xml.apache.org/xslt"
xmlns:Math="xalan://java.lang.Math">
   
<xalan:component prefix="Math" functions="sin cos tan atan">
 <xalan:script lang="javaclass" src="xalan://java.lang.Math"/>
</xalan:component>
   
<xsl:variable name="PI" select="4.0 *"/>

If you use this form and omit the xalan:component element, then your stylesheet can work with both Saxon and Xalan.