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Jul 1, 2020 at 11:35 comment added Just dropped in $\mathbb{Q}$ are the rational numbers. Instead of taking the linear span with real coefficients, take the linear span with rational coefficients. this is dense and $\mathbb{Q}$ is countable, which will make the $Q$-span of a countable set countable as well.
Jul 1, 2020 at 11:16 comment added fina Last doubt @JustDroppedIn, $\mathbb{Q}-span$, I don't understand this. What is $\mathbb{Q}$?
Jul 1, 2020 at 9:56 comment added Just dropped in @fina I'm afraid I don't have what it takes to discuss these concepts. Sorry!
Jul 1, 2020 at 9:44 comment added Just dropped in @fina I am confused with these notations, I can't understand what are $X_i(t)$ and what is $L_p$ here?
Jul 1, 2020 at 9:32 comment added Just dropped in @fina Yes, you can understand these vectors as infinite column matrices and these are indeed scalars. Hilbert dimension is defined in my answer: it is the number of elements that an orthonormal basis has. I hope this helps!
Jul 1, 2020 at 9:32 vote accept fina
Jul 1, 2020 at 9:30 comment added fina But then $\langle x,e_{i}\rangle=x_{i}$ are scalar variables, which can be notated either as an infinite matrix, or an infinite vector column, am I right? On the other hand, what is the meaning of "Hilbert dimension"? Do you refer to the dimension of the Hilbert space? And then, I understand that the "nature" of the dimension is given from an index set. However, I don't have that capacity to understand all! But, anyway, fantastic answer!
Jul 1, 2020 at 8:38 history answered Just dropped in CC BY-SA 4.0