Vector Spaces
A vector space is the central object of study in linear algebra. It abstracts the familiar properties of arrows in the plane to a far more general setting.
Definition
A vector space over a field is a set together with two operations:
- Addition: , denoted
- Scalar multiplication: , denoted
satisfying the following axioms for all and :
- (commutativity)
- (associativity)
- There exists such that (zero vector)
- For each , there exists such that (additive inverse)
- (compatibility)
- (identity)
- (distributivity over vector addition)
- (distributivity over scalar addition)
The elements of are called vectors and the elements of are called scalars. Common choices for are (real vector spaces) and (complex vector spaces). When the field is clear from context, we simply say " is a vector space."
Fundamental examples
with componentwise addition and scalar multiplication:
The zero vector is . This is the prototypical vector space.
is a vector space over with the same operations as . Note that can also be viewed as a vector space over (of dimension ).
The set of all matrices with entries in , with the usual matrix addition and scalar multiplication, forms a vector space over . The zero vector is the zero matrix.
is the space of polynomials of degree at most . With the usual addition and scalar multiplication of polynomials, is a vector space over .
is the space of all polynomials, which is an infinite-dimensional vector space.
Let be any nonempty set. The set of all real-valued functions on is a vector space with pointwise operations:
The zero vector is the zero function for all .
is a vector space over . The sum of continuous functions is continuous, and a scalar multiple of a continuous function is continuous. This is a subspace of .
consisting of just the zero vector is a vector space over any field . It is the smallest possible vector space, with .
Any field is a vector space over itself, with scalar multiplication being the field multiplication. More generally, if is a field extension, then is a vector space over . For instance, is a 2-dimensional vector space over , with basis .
The set of all infinite sequences is a vector space over with componentwise operations. Notable subspaces include:
- : sequences with (a Hilbert space)
- : sequences converging to
- : sequences with only finitely many nonzero terms
The set of all solutions to a homogeneous linear system (where is an matrix) is a vector space over : if and , then and . This is a subspace of .
is a vector space over with basis . Every complex number is a unique linear combination of and with real coefficients. Thus .
is a vector space over . However, this vector space is infinite-dimensional (in fact, uncountably so). A basis is called a Hamel basis, and its existence requires the axiom of choice.
Basic properties
In any vector space over :
- The zero vector is unique.
- The additive inverse is unique for each .
- for all (scalar zero gives vector zero).
- for all .
- for all .
- If , then or .
For (3): . Adding to both sides gives .
For (5): , so is the additive inverse of .
For (6): Suppose and . Then exists in , and .
A vector space is an abelian group equipped with a compatible scalar multiplication by a field . More generally, replacing the field by a ring gives the notion of an -module. Modules over rings are central in commutative algebra and algebraic geometry.