Hilbert Spaces – Example of Non-Closed Subspace of a Hilbert Space

Question

I am reading a book on Hilbert space.

It seems that the author assumes that a linear subspace of a Hilbert space can be non-closed.

I cannot think of an example. I am still used to the finite-dimensional case.

Can anyone give me an example?

Answer 1

Consider the space of square summable sequences and the subspace of sequences whose only finitely many terms are different from zero. This subspace is dense and is not the whole space hence it cannot be closed.