I studyng looked in a book of Barbu a notation of type "Cauchy Problem $$f(t) \in \frac{dy(t)}{dt}+Ay(t), \ t \in [0,T]\\ \\ y(0)=y_0 $$ $ f \in L^1(0,T;X)$". Why the author used $\in$ and no $=$?
2026-03-28 02:21:27.1774664487
Why use the notation $\in$ in Cauchy Problem?
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