master
<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>