Display:

$\frac{\partial^{2}f(x, y)}{\partial x\partial y}$

Source:

<apply>
     <partialdiff/>
     <bvar>
          <ci> x </ci>
     </bvar>
     <bvar>
          <ci> y </ci>
     </bvar>
     <apply>
          <ci type="fn"> f </ci>
          <ci> x </ci>
          <ci> y </ci>
     </apply>
</apply>