Improve speed of inv by using the cache.

src/sage/data_structures/coefficient_stream.py

@@ -587,18 +587,19 @@ def iterate_coefficients(self):
  """
  Return the generator for the coefficients of the multiplicative inverse of the ``series``.
  """
- n = self._offset
+ v = self._approximate_valuation # shorthand name
+ n = 0 # Counts the number of places from the valuation
+ yield self._ainv
+ # Note that first entry of the cache will correspond to z^v
  while True:
- v = self._approximate_valuation
- if n == v:
- yield self._ainv
- n += 1
- continue
+ n += 1
  c = self._zero
- for k in range(v, n):
- c += self[k] * self._series[n - v - k]
+ m = min(len(self._cache), n)
+ for k in range(m):
+ c += self._cache[k] * self._series[n - k]
+ for k in range(m, n):
+ c += self[v+k] * self._series[n - k]
  yield -c * self._ainv
- n += 1
 
 
 class CoefficientStream_apply_coeff(LazyLaurentSeries_unary):