<?xml version="1.0" encoding="UTF-8"?>
<math xmlns="http://www.w3.org/1998/Math/MathML">
   <mi>f</mi>
   <mo>:</mo>
   <mrow>
      <msub>
         <mi mathvariant="double-struck">N</mi>
         <mn>1</mn>
      </msub>
   </mrow>
   <mrow>
      <msub>
         <mi mathvariant="double-struck">N</mi>
         <mn>1</mn>
      </msub>
   </mrow>
</math>
