title: scipy.interpolate.make_interp_spline how to retrieve all the coefficients?
body: <p>I have the following program:</p>
  <pre><code>nknots = 4
  x_i = [0, 1, 2, 3]
  y_i = [1, np.exp(1), np.exp(2), np.exp(3)]
  coeff = interpolate.make_interp_spline(x_i, y_i, bc_type=&quot;natural&quot;)
  </code></pre>
  <p>I want to construct a cubic spline using the knots whose coordinates are given by x_i and y_i arrays. However, I'm having a hard time obtaining all the coefficients. A cubic spline function has the following form:</p>
  <pre><code>y_i(x) = a + b*(x - x_i) + c*(x - x_i)^2 + d*(x - x_i)^3
  </code></pre>
  <p>When I do</p>
  <pre><code>print(coeff(x_i))
  </code></pre>
  <p>I get only the array of <em>a</em> values:</p>
  <pre><code>[ 1.          2.71828183  7.3890561  20.08553692]
  </code></pre>
  <p>However, I'm missing the arrays for the <em>b</em>, <em>c</em>, and <em>d</em> coefficients. How do I extract those? Or are there steps I'm missing? I read the scipy documentation on make_interp_spline but I didn't understand how to get the <em>b</em>, <em>c</em> and <em>d</em> coefficients.</p>