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$\left(\genfrac{}{}{0ex}{}{a}{b}\right)$

Source Code:

    <math> 
      <mrow> 
        <mo> ( </mo> 
        <mfrac linethickness="0"> 
          <mi> a </mi> 
          <mi> b </mi> 
        </mfrac> 
        <mo> ) </mo> 
      </mrow> 
    </math>