title: By two combinations of predictors in linear regression in R
body: <p>Suppose that I have <code>X1,...,X14</code> potential predictors. </p>
  
  <p>Now for a given <code>Y</code> i want to make the OLS scheme:</p>
  
  <pre><code>Y~X1+X2
  Y~X1+X3
   ....
  Y~X1+X14
  ....
  Y~X14+X13
  </code></pre>
  
  <p>which is basically all the by two combinations of all the predictors. After all those regressions are created I want to use them in the <code>predict</code> function (if possible). </p>
  
  <p>My question is: How do i make all those regressions with all by two combinations of the regressors?</p>