title: SymPy fails limit test
body: <p>I am using the ratio test to find if the series with element <code>a = n**2/3**n</code> converges. On paper it is easy to see that the ratio a(n+1)/a(n) -> 1/3, but SymPy seem to fail in this test,</p>
  
  <pre><code>&gt;&gt;&gt; n = symbols('n', integer=True)
  &gt;&gt;&gt; a = n**2/3**n
  &gt;&gt;&gt; a1 = a.subs(n, n+1)
  &gt;&gt;&gt; r = a1/a
  &gt;&gt;&gt; r
  3**n*3**(-n - 1)*(n + 1)**2/n**2
  &gt;&gt;&gt; simplify(r)
  (n + 1)**2/(3*n**2)
  &gt;&gt;&gt; expand(r)
  1/3 + 2/(3*n) + 1/(3*n**2)
  &gt;&gt;&gt; limit(r, n, oo)
  1
  &gt;&gt;&gt; limit(expand(r), n, oo)
  1/3
  </code></pre>
  
  <p>Why can't SymPy evaluate the correct limit on the ratio a1/a ?</p>