Let XX be a real .

Proposition: A subset ΩX\Omega\subset X is a if and only if there exist a nonzero linear functional f:XRf:X\to\mathbb{R} and a scalar αR\alpha\in\mathbb{R} such that

Ω={xXf(x)=α}.\Omega=\{x\in X\mid f(x)=\alpha\}.
Remarks

Context: One direction uses the decomposition of codimension-one subspaces (see ). The other direction uses that kerf\ker f has codimension one (see ).