What is meant by
the integral is interpreted in the weak sense
on page 261 of the book "An introduction to frame and Riesz bases", second edition, by Ole Christenson.
There is no an explanation in the book.
What is meant by
the integral is interpreted in the weak sense
on page 261 of the book "An introduction to frame and Riesz bases", second edition, by Ole Christenson.
There is no an explanation in the book.
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An integral is a limit. A limit depends on the topology you use. In a Hilbert space as $L^2(\mathbb R)$, the two most natural topologies are the strong one, where $f_j\to f$ means $\langle f_j-f,f_j-f\rangle\to0$ (that is, the one given by the norm), and the weak one, where $f_j\to f$ means $$ \langle f_j-f,g\rangle\to0\ \ \ \ \text{ for all } g\in L^2(\mathbb R). $$