<math
xmlns='http://www.w3.org/1998/Math/MathML'>
<semantics>
<msub>
<mi>b</mi>
<mn>3</mn>
</msub>
<annotation-xml encoding='MathML-Content'>
<apply>
<ci>Subscript</ci>
<ci>b</ci>
<cn type='integer'>3</cn>
</apply>
</annotation-xml>
</semantics>
</math>