title: pandas - return column of exponential values
body: <p>Starting from a sample dataframe <code>df</code> like:</p>
  
  <pre><code>a,b
  0,0.71
  1,0.75
  2,0.80
  3,0.90
  </code></pre>
  
  <p>I would add a new column with exponential values of column <code>b</code>. So far I tried:</p>
  
  <pre><code>df['exp'] = math.exp(df['b'])
  </code></pre>
  
  <p>but this method returns:</p>
  
  <pre><code>"cannot convert the series to {0}".format(str(converter)"
  TypeError: cannot convert the series to &lt;type 'float'&gt;
  </code></pre>
  
  <p>Is there a way to apply a <code>math</code> function to a whole column?</p>