<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <semantics>
    <mrow>
      <msub>
        <mi>A</mi>
        <mfenced close="" open="">
          <mi>i</mi>
          <mi>j</mi>
        </mfenced>
      </msub>
      <mo>&InvisibleTimes;</mo>
      <msub>
        <mi>x</mi>
        <mi>j</mi>
      </msub>
    </mrow>
    <annotation-xml encoding="MathML-Content">
      <apply>
        <times/>
        <ci>
          <msub>
            <mi>A</mi>
            <mfenced close="" open="">
              <mi>i</mi>
              <mi>j</mi>
            </mfenced>
          </msub>
        </ci>
        <ci>
          <msub>
            <mi>x</mi>
            <mi>j</mi>
          </msub>
        </ci>
      </apply>
    </annotation-xml>
    <annotation encoding="SnuggleTeX">\[ A_{i,j}x_j \]</annotation>
    <annotation encoding="LaTeX">\[ A_{i,j}x_j \]</annotation>
    <annotation encoding="Maxima">A[i,j] * x[j]</annotation>
  </semantics>
</math>

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <semantics>
    <mrow>
      <msub>
        <mi>A</mi>
        <mrow>
          <mi>i</mi>
          <mo>,</mo>
          <mi>j</mi>
        </mrow>
      </msub>
      <msub>
        <mi>x</mi>
        <mi>j</mi>
      </msub>
    </mrow>
    <annotation encoding="SnuggleTeX">\[ A_{i,j}x_j \]</annotation>
  </semantics>
</math>

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mrow>
    <msub>
      <mi>A</mi>
      <mfenced close="" open="">
        <mi>i</mi>
        <mi>j</mi>
      </mfenced>
    </msub>
    <mo>&InvisibleTimes;</mo>
    <msub>
      <mi>x</mi>
      <mi>j</mi>
    </msub>
  </mrow>
</math>

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <apply>
    <times/>
    <ci>
      <msub>
        <mi>A</mi>
        <mfenced close="" open="">
          <mi>i</mi>
          <mi>j</mi>
        </mfenced>
      </msub>
    </ci>
    <ci>
      <msub>
        <mi>x</mi>
        <mi>j</mi>
      </msub>
    </ci>
  </apply>
</math>

A[i,j] * x[j]