4.3 Linearly Independent Sets; Bases 4 Vector Spaces 4.3 Linearly Independent Sets; Bases
REVIEW Definition A vector space is a nonempty set V of objects, called vectors, on which are defined two operations, called addition and multiplication by scalars, subject to the ten axioms:
REVIEW Definition A subspace of a vector space V is a subset H of V that satisfies The zero vector of V is in H. H is closed under vector addition. H is closed under multiplication by scalars.
REVIEW Theorem 1 If are in a vector space V, then Span is a subspace of V.
REVIEW Definition The null space of an matrix A, written as Nul A, is the set of all solutions to Ax=0. Theorem 2 The null space of an matrix A is a subspace of .
REVIEW Definition The column space of an matrix A, written as Col A, is the set of all linear combinations of the columns of A. Note: Theorem 3 The column space of an matrix A is a subspace of .
REVIEW Definition: A linear transformation T from a vector space V into a vector space W is a rule assigns to each vector x in V a unique vector T(x) in W, such that (i) T(u+v)=T(u)+T(v) for all u, v in the domain of T: (ii) T(cu)=cT(u) for all u and all scalars c. If T(x)=Ax for some matrix A, Kernel of T = Nul A Range of T = Col A.
4.3 Linearly Independent Sets; Bases Purpose: To study the vectors that span a vector space (or a subspace) as efficiently as possible.
Linear Independence in V is linearly independent has only the trivial solution.
Tips to determine the linear dependence A set is linearly dependent, if it satisfies one of the following: A set has two vectors and one is a multiple of the other. 2. A set has two or more vectors and one of the vectors is a linear combination of the others. 3. A set contains more vectors than the number of entries in each vector. A set contains the zero vector.
Theorem 4 An indexed set of two or more vectors, with , is linearly dependent if and only if some is a linear combination of the preceding vectors, Example:
Definition Let H be a subspace of a vector space V. An indexed set of vectors in V is a basis for H if i) is a linearly independent set, and ii) the subspace spanned by coincides with H; i.e.
Examples: 1. Let A be an invertible matrix. Then the columns of A form a basis for . Why? 2. Let be the columns of the identity matrix I. Then, is called the standard basis for .
4. Let . Determine if is a basis for . 5. Let Determine if is a basis for . 6. Let Determine if is a basis for .
The Spanning Set Theorem Let be a set in V, and let . If one of the vectors in S, say , is a linear combination of the remaining vectors in S, then the set formed from S by removing still spans H. b. If , some subset of S is a basis for H.
Example: Find a basis for Col A, where
Example: Find a basis for Col B, where
Theorem The pivot columns of a matrix A form a basis for Col A.