Yes, if the document type you use provides for math, and your users’ browsers are capable of rendering it. The mathematics-using community has developed the MathML Recommendation at the W3C, which is a native XML application suitable for embedding in other DTDs and Schemas. It is also possible to make XML fragments from other DTDs, such as ISO 12083 Math, or OpenMath, or one of your own making. Browsers which display math embedded in SGML existed for many years (eg DynaText, Panorama, Multidoc Pro), and mainstream browsers are now rendering MathML. David Carlisle has produced a set of stylesheets for rendering MathML in browsers. It is also possible to use XSLT to convert XML math markup to LATEX for print (PDF) rendering, or to use XSL:FO.
Please note that XML is not itself a programming language, so concepts such as arithmetic and if-statements (if-then-else logic) are not meaningful in XML documents.