title: How to write equation including `abs` inside, using Rmarkdown?
body: <p>I have managed to write the whole equation
  <a href="https://i.stack.imgur.com/onmO4.png" rel="noreferrer"><img src="https://i.stack.imgur.com/onmO4.png" alt="Equation I want to write"></a></p>
  
  <p>using the following code in Rmarkdown</p>
  
  <pre><code>$$MAPE = \frac{1}{n} \sum_{d_i} (\frac{1}{q} \sum_{t_j} 
  \abs{\frac{gap_{i,j}-s_{i,j}}{gap_{i,j}}}  )$$ 
  </code></pre>
  
  <p>However, in Rmarkdown, the code above returns:</p>
  
  <p><a href="https://i.stack.imgur.com/neU5G.png" rel="noreferrer"><img src="https://i.stack.imgur.com/neU5G.png" alt="Rmarkdown output"></a></p>
  
  <p>Could anyone help figure out how to get <code>abs</code> right here?
  Thanks</p>