imaginary i
<math xmlns="http://www.w3.org/1998/Math/MathML">
<apply>
<csymbol cd="relation1">eq</csymbol>
<apply>
<csymbol cd="arith1">power</csymbol>
<csymbol cd="nums1">i</csymbol>
<cn>2</cn>
</apply>
<cn>-1</cn>
</apply>
</math>
Author: MathML 2 Recommendation