Orthogonal complements in prehilbert spaces?
1475
In an incomplete inner product space, is a closed linear subspace equal to the orthogonal complement of its orthogonal complement?
7069
On the product of $\mathfrak c$-many separable spaces
145952
Example of a closed subspace of a Banach space which is not complemented?
154625
$\operatorname{span}(x^0, x^1, x^2,\cdots)$ and the vector space of all real valued continuous functions on $\Bbb R$
128835
Compactness of the set of $n \times n$ orthogonal matrices
54377
Complementability of von Neumann algebras
154628
What should we understand from the definition of orthogonality in inner product spaces other than $\mathbb R^n$?
32420
A complete orthonormal system contained in a dense sub-space.
67191
When is the union of a family of subspaces of a vector space also a subspace?
35625
Direct sum of orthogonal subspaces
45998
The range of $T^*$ is the orthogonal complement of $\ker(T)$
74884
Tensor product in dual-space
114139
Convex Hull of Precompact Subset is Precompact
129194
Correspondences between Borel algebras and topological spaces
77563
Homeomorphism between spaces equipped with cofinite topologies
104607
Hahn-Banach theorem, dual space
133711
Orthogonal basis for infinite-dimensional vector spaces