title: Evaluating a sum in SymPy
body: <p>I want to evaluate the sum</p>
  <p><img src="https://latex.codecogs.com/gif.latex?%5Csum_%7Bn=1%7D%5E%5Cinfty&space;e%5E%7B-nx%7D&space;=&space;%5Cfrac%7B1%7D%7Be%5Ex-1%7D" title="\sum_{n=1}^\infty e^{-nx} = \frac{1}{e^x-1}" /></p>
  <p>using Python and SymPy, I'm relatively new using SymPy and the first thing I tried was</p>
  <pre><code>from sympy import exp, oo, summation, symbols
  
  n, x = symbols('n,x')
  summation(exp(-n*x), (n, 1, oo))
  </code></pre>
  <p>But it doesn't work, it just returns the unevaluated sum.
  I can make it work using</p>
  <pre><code>from sympy import exp, oo, summation, symbols
  
  n, x = symbols('n,x')
  f = exp(-x)
  summation(x**(-n), (n, 1, oo)).subs(x,f)
  </code></pre>
  <p>But I would like to know if it is possible to make it work without need to break the expression into x^n and then substitute x by e^-x.</p>
  <p>Thank you</p>