title: SARIMAX model in R
body: <p>I would fit a <code>SARIMAX</code> model with temperature as exogenous variable in <code>R</code>. Can I do that with <code>xreg</code> function present in the <code>package TSA</code>? 
  I thought to fit the model as:</p>
  
  <p><code>fit1 = arima(x, order=c(p,d,q), seasonal=list(order=c(P,D,Q), period=S), xreg=temp)</code> </p>
  
  <p>is that correct or I have to use other function of <code>R</code>?
  if it itsn't correct: which steps should I use?</p>
  
  <p>Thanks.</p>