Power series:
  Finding radius of covergence
Applying ratio test and thinking that
"limit of ratio of coefficients = radius of convergence"
(it's actually the reciprocal)
-- it's worth pointing out that this always works;
it's actually a theorem, and worth understanding.
(whose theorem is it again?)

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ratio test:
  if the ratio is always < 1,
  but not bounded away from 1, then you can't apply the ratio/root test!
