<math xmlns="http://www.w3.org/1998/Math/MathML">
<apply>
<csymbol cd="set1">cartesian_product</csymbol>
<csymbol cd="setname1">R</csymbol>
<csymbol cd="setname1">R</csymbol>
<csymbol cd="setname1">R</csymbol>
</apply>
</math>
Author: MathML 2 Recommendation