<?xml version="1.0" encoding="UTF-8"?>

<math xmlns="http://www.w3.org/1998/Math/MathML">
  <mfenced>
    <msub>
      <mi>a</mi> 
      <mn>1</mn> 
    </msub> 
    <msub>
      <mi>a</mi> 
      <mn>2</mn> 
    </msub> 
    <msub>
      <mi>a</mi> 
      <mn>3</mn> 
    </msub> 
  </mfenced> </math>