title: Meaning of negative axis parameter in sparse matrix functions
body: <p>According to the <a href="https://docs.scipy.org/doc/scipy/reference/generated/scipy.sparse.csr_matrix.max.html#scipy.sparse.csr_matrix.max" rel="nofollow noreferrer">docs</a> the <code>max</code> function of a sparse matrix accepts negative values as axis parameter:</p>
  
  <blockquote>
    <p>Parameters: axis : {-2, -1, 0, 1, None} optional</p>
  </blockquote>
  
  <p>Following the links in the documentation to <a href="https://docs.scipy.org/doc/numpy-1.13.0/reference/generated/numpy.amax.html" rel="nofollow noreferrer">numpy.amax</a> I can find examples for the two positive values <code>0</code> and <code>1</code>:</p>
  
  
  
  <pre class="lang-py prettyprint-override"><code>&gt;&gt;&gt; a
  array([[0, 1],
         [2, 3]])
  &gt;&gt;&gt; np.amax(a, axis=0)   # Maxima along the first axis
  array([2, 3])
  &gt;&gt;&gt; np.amax(a, axis=1)   # Maxima along the second axis
  array([1, 3])
  </code></pre>
  
  <p>What is the meaning of the two negative values <code>-1</code>and <code>-2</code>?</p>