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An affine set whose direction subspace has codimension one.
Let be a vector space. An affine set is called a hyperplane if it has codimension one.
More precisely: if , it is parallel to a unique subspace (see Ω−Ω characterization). Then is a hyperplane iff .
In real vector spaces, hyperplanes are exactly level sets of nonzero linear functionals; see the level-set characterization.
A vector space over a field is a set with operations (functions) and , and an element , satisfying the following for all and :
Let be a vector space and let be a linear subspace. The codimension of in is
where is the quotient space of by .
Codimension measures "how many independent linear constraints" define . Codimension one subspaces play a special role in the geometry of hyperplanes.
Let be a vector space and let be a nonempty affine set.
Proposition: The set is parallel to a unique linear subspace , and this subspace is
Context: The subspace captures the "direction" of the affine set. It is the appropriate linear object used to define hyperplanes as affine sets of codimension one.
Let be a real vector space.
Proposition: A subset is a hyperplane if and only if there exist a nonzero linear functional and a scalar such that