<?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>