Display:

$\frac{\partial^{k}f(x, y)}{\partial x^m\partial y^n}$

Source:

<apply>
     <partialdiff/>
     <bvar>
          <ci> x </ci>
          <degree>
               <ci> m </ci>
          </degree>
     </bvar>
     <bvar>
          <ci> y </ci>
          <degree>
               <ci> n </ci>
          </degree>
     </bvar>
     <degree>
          <ci> k </ci>
     </degree>
     <apply>
          <ci type="fn"> f </ci>
          <ci> x </ci>
          <ci> y </ci>
     </apply>
</apply>