title: Sympy summation with indexed variable
body: <p>I try to create a sympy expression with a Sum with an indexed variable as previous explain <a href="https://stackoverflow.com/questions/24834447/how-to-create-an-indexed-variable-in-sympy">here</a>
  However, I can not do lambdify of this expression and give an array to get the sum calculated.
  Is this impossible?</p>