title: Python Error only length-1 arrays can be converted to Python scalars
body: <p>I am new to python coding
  and i am writing a code to calculate the sum of a series, i started by writing a function where the input is the number of iterations of the sum, but when i compile it gives me the error in the line under <code>def G(i)</code> : only length-1 arrays can be converted to Python scalars</p>
  
  <p>Can you help me please</p>
  
  <pre><code>import matplotlib.pyplot as plt
  import numpy as np
  import scipy.special as sp
  import pylab as pylab
  
  def G(i):
      return (sum(((-1*(2*l+1))/(4*np.pi*(l**2+l)))*sp.legendre(l)(0.5)  for l in i))
  
  pylab.ylim([-1,1])
  sumrange = np.arange(1,70,1)
  plt.plot(sumrange,G(sumrange),color='red')
  </code></pre>