__Suggestions in Differential Equations__

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Suggestions are inspirations based on similarities. They
are an indispensible part of mathematical thinking. They are neither formal arguments nor results derived from quoting theorems. We may provide reasons to support them but not proofs.
- [Cod, p.139, (1.7)] is suggested by the the variation-of-constants formula, but is not derived from it because
jis unknown. We may use either (e
^{-it}, e^{it}) or (e^{it}, e^{-it}) as the first row of
F(t) in [Cod, p.75, (3.2)], but only the former choice may lead to [Cod, p.139, (1.7)].

- [Cod, p.139, (1.4)] is suggested by [Cod, p.89, (6.29); p.133, l.6].