__Viewpoints in Differential Equations__

- It is difficult to prove Abel's theorem [Ru1, p.160, Theorem 8.2] from the
viewpoint of series. However, it is easy to understand the origin of Abel's theorem from the
viewpoint of analytic continuation [Guo, p.522, l.11-l.12].

- We produce the good viewpoint [Perr, p.11, (17)] by comparing [Perr, p.11, (15)] with [Perr, p.11, (16)].

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