title: Sympy Symbolic Matrix Exponential
body: <p>I am am trying to find out, if I can somehow find a matrix exponential in kind of closed form for the symbolic matrix:</p>
  
  <pre><code>n=3;
  a = symbols(['a'+str(1+k) for k in range(n)], real=True);
  Ts = symbols('T_s',real=True,positive=True);
  A = Matrix([zeros(1,n),eye(1,n),a])
  </code></pre>
  
  <p>however</p>
  
  <pre><code>expm(A)
  </code></pre>
  
  <p>seems not to work, "TypeError: cannot create mpf from a1"!
  Then I tried to get an estimate using the series expansion, but again:</p>
  
  <pre><code>Sum(A**n, (n, 0, 1))
  </code></pre>
  
  <p>gives: "ValueError: Matrix det == 0; not invertible". I guess they try to get the power of the matrix by joran blocks and my structure is not good for that. How can I proceed anyway? <code>A**2</code> can be calculated, but the sum over <code>A**1</code> and <code>A**0</code> not?!</p>