title: Sympy: get a column of a matrix power
body: <p>This seems to be a simple question. My code reads</p>
  
  <pre><code>import sympy as sp
  D1 = sp.symbols('D1')
  D2 = sp.symbols('D2')
  m1 = sp.diag(*[2 * D1, 2 * D1, 2 * D2, 2 * D2])
  b = sp.sqrt(m1)
  
  m1.col(0)
  b.col(0)
  </code></pre>
  
  <p>I.e. I want to get a column of the square root of a matrix. However, it seems like the <code>MatPow</code> object <code>b</code> does not have the <code>.col</code> attribute, which normal matrices like <code>m1</code> have.</p>
  
  <p>Output:</p>
  
  <pre><code>AttributeError                            Traceback (most recent call last)
  &lt;ipython-input-55-855c6e53a412&gt; in &lt;module&gt;()
  ----&gt; 1 b.col(0)
  
  AttributeError: 'MatPow' object has no attribute 'col'
  </code></pre>
  
  <p>What am I doing wrong?</p>