No Nonzero multiplication operator is compact
74147
Compactness of  Multiplication Operator on $L^2$
102585
Why is $L^{\infty}$ not separable?
110395
Integrable function $f$ on $\mathbb R$ does not imply that limit $f(x)$ is zero
95481
The set of self-adjoint operators over a Hilbert space doesn't form a lattice
80202
Why is $[0, 1] \cap \mathbb{Q}$ not compact in $\mathbb{Q}$?
71011
Criteria of compactness of an operator
12189
Prove norm doesn't come from inner product.
71945
How to reduce circuit elements of a decomposed $C^2(U)$ operation?
104097
An idempotent operator is compact if and only if it is of finite rank
18160
How to show that the group of nonzero real numbers under multiplication is not isomorphic to the group of real numbers under addition?
4717
The union of linear subspaces is not necessarily a subspace
120475
Vector spaces - Multiplying by zero scalar yields zero vector
29474
Compactness of $\operatorname{Spec}(A)$
121198
$W^{1,1}(\mathbb{R}^n)$ is not compactly embedded in $L^1(\mathbb{R}^n)$?
87580
Canonical examples of inner product spaces that are not Hilbert spaces?
45249
Pseudocompactness does not imply compactness
12467
Why does this proof work: Closed unit ball in $C_0$ is not compact