# IMPORTANT DEFINITIONS AND THEOREMS REFERENCE SHEET

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1 IMPORTANT DEFINITIONS AND THEOREMS REFERENCE SHEET This is a (not quite comprehensive) list of definitions and theorems given in Math Pay particular attention to the ones in red. Study Tip For each definition, find an example of something that satisfies the requirements of the definition, and an example of something that does not. For each theorem, find an example of something that satisfies the hypotheses of the theorem, and an example of something that does not satisfy the conclusions (or the hypotheses, of course) of the theorem. This is great conceptual practice. CHAPTER 1 Definition. R n = all ordered n-tuples of real numbers (x 1, x 2,..., x n ). SECTION 1.1. Definition. A solution to a system of linear equations is a list of numbers making all of the equations true. Definition. The elementary row operations are the following matrix operations: Multiply all entries in a row by a nonzero number (scale). Add (a multiple of) each entry of one row to the corresponding entry in another (row replacement). Swap two rows. Definition. Two matrices are called row equivalent if one can be obtained from the other by doing some number of elementary row operations. Definition. A system of equations is called inconsistent if it has no solution. It is consistent otherwise. SECTION 1.2. Definition. A matrix is in row echelon form if (1) All zero rows are at the bottom. (2) Each leading nonzero entry of a row is to the right of the leading entry of the row above. (3) Below a leading entry of a row, all entries are zero. Definition. A pivot is the first nonzero entry of a row of a matrix in row echelon form. 1

2 2 DEFINITIONS AND THEOREMS Definition. A matrix is in reduced row echelon form if it is in row echelon form, and in addition, (4) The pivot in each nonzero row is equal to 1. (5) Each pivot is the only nonzero entry in its column. Theorem. Every matrix is row equivalent to one and only one matrix in reduced row echelon form. Definition. Consider a consistent linear system of equations in the variables x 1,..., x n. Let A be the reduced row echelon form of the matrix for this system. We say that x i is a free variable if its corresponding column in A is not a pivot column. Definition. The parametric form for the general solution to a system of equations is a system of equations for the non-free variables in terms of the free variables. For instance, if x 2 and x 4 are free, x 1 = 2 3x 4 x 3 = 1 4x 4 is a parametric form. Theorem. Every solution to a consistent linear system is obtained by substituting (unique) values for the free variables in the parametric form. Fact. There are three possibilities for the solution set of a linear system with augmented matrix A: (1) The system is inconsistent: it has zero solutions, and the last column of A is a pivot column. (2) The system has a unique solution: every column of A except the last is a pivot column. (3) The system has infinitely many solutions: the last column isn t a pivot column, and some other column isn t either. These last columns correspond to free variables. SECTION 1.3. Definition. A point is an element of R n, drawn as a point (a dot). A vector is an element of R n, drawn as an arrow. Definition. A scalar is another name for a real number (to distinguish it from a vector). Review. Parallelogram law for vector addition. Definition. A linear combination of vectors v 1, v 2,..., v n is a vector of the form c 1 v 1 + c 2 v c n v n where c 1, c 2,..., c n are scalars, called the weights or coefficients of the linear combination. Definition. A vector equation is an equation involving vectors. (It is equivalent to a list of equations involving only scalars. It is also equivalent to an augmented matrix.) Definition. The span of a set of vectors v 1, v 2,..., v n is the set of all linear combinations of these vectors: Span{v 1,..., v p } = x 1 v x p v p x1,..., x p in R.

3 Theorem. The following are equivalent: (1) A vector b is in the span of v 1, v 2,..., v p. (2) The vector equation DEFINITIONS AND THEOREMS 3 x 1 v 1 + x 2 v x p v p = b has a solution. (3) The linear system with augmented matrix v 1 v 2 v p b SECTION 1.4. is consistent. Definition. The product of an m n matrix A with a vector x in R n is the linear combination x 1 x Ax = v 1 v 2 v n 2. := x 1v 1 + x 2 v x n v n. The output is a vector in R m. x n Definition. A matrix equation is a vector equation involving a product of a matrix with a vector. Theorem. Ax = b has a solution if and only if b is in the span of the columns of A. Theorem. Let A be an m n (non-augmented) matrix. The following are equivalent (1) Ax = b has a solution for all b in R m. (2) The span of the columns of A is all of R m. (3) A has a pivot in each row. SECTION 1.5. Definition. A system of linear equations of the form Ax = is called homogeneous. Definition. A system of linear equations of the form Ax = b for b is called inhomogeneous or non-homogeneous. Definition. The trivial solution to a homogeneous equation is the solution x = : A =. Theorem. Let A be a matrix. The following are equivalent: (1) Ax = has a nontrivial solution. (2) There is a free variable. (3) A has a column with no pivot. Theorem. The solution set of a homogeneous equation Ax = is a span.

4 4 DEFINITIONS AND THEOREMS Definition. The parametric vector form for the general solution to a system of equations Ax = b is a vector equation expressing all variables in terms of the free variables. For instance, if x 2 and x 4 are free, x 1 x 2 x = x = 3 x x 2 + x is a parametric vector form. The constant vector (2,, 1, ) is a specific solution or particular solution to Ax = b. Theorem. The solution set of a linear system Ax = b is a translate of the solution set of Ax = by a specific solution. SECTION 1.7. Definition. A set of vectors {v 1, v 2,..., v p } in R n is linearly independent if the vector equation x 1 v 1 + x 2 v x p v p = has only the trivial solution x 1 = x 2 = = x p =. Definition. A set of vectors {v 1, v 2,..., v p } in R n is linearly dependent if the vector equation x 1 v 1 + x 2 v x p v p = has a nontrivial solution (not all x i are zero). Such a solution is a linear dependence relation. Theorem. A set of vectors {v 1, v 2,..., v p } is linearly dependent if and only if one of the vectors is in the span of the other ones. Fact. Say v 1, v 2,..., v n are in R m. If n > m then {v 1, v 2,..., v n } is linearly dependent. Fact. If one of v 1, v 2,..., v n is zero, then {v 1, v 2,..., v n } is linearly dependent. Theorem. Let v 1, v 2,..., v n be vectors in R m, and let A be the m n matrix with columns v 1, v 2,..., v n. The following are equivalent: (1) The set {v 1, v 2,..., v n } is linearly independent. (2) No one vector is in the span of the others. (3) For every j between 1 and n, v j is not in Span{v 1, v 2,..., v j 1 }. (4) Ax = only has the trivial solution. (5) A has a pivot in every column. Theorem. Suppose that {v 1, v 2,..., v p } is linearly independent. Then every vector w in Span{v 1, v 2,..., v p } can be written in exactly one way as a linear combination w = a 1 v 1 + a 2 v a p v p.

5 DEFINITIONS AND THEOREMS 5 SECTION 1.8. Definition. A transformation (or function or map) from R n to R m is a rule T that assigns to each vector x in R n a vector T(x) in R m. R n is called the domain of T. R m is called the codomain of T. For x in R n, the vector T(x) in R m is the image of x under T. Notation: x T(x). The set of all images {T(x) x in R n } is the range of T. Notation. T : R n R m means T is a transformation from R n to R m. Definition. Let A be an m n matrix. The matrix transformation associated to A is the transformation T : R n R m defined by T(x) = Ax. The domain is R n, where n is the number of columns of A. The codomain is R m, where m is the number of rows of A. The range is the span of the columns of A. Review. Geometric transformations: projection, reflection, rotation, dilation, shear. Definition. A linear transformation is a transformation T satisfying for all vectors u, v and all scalars c. T(u + v) = T(u) + T(v) and T(cv) = ct(v) SECTION 1.9. Definition. The unit coordinate vectors in R n are 1 e 1 =., e 2 = 1.,..., e n 1 =., e n = Fact. If A is a matrix, then Ae i is the ith column of A. Definition. Let T : R n R m be a linear transformation. The standard matrix for T is T(e 1 ) T(e 2 ) T(e n ). Theorem. If T is a linear transformation, then it is the matrix transformation associated to its standard matrix. Definition. A transformation T : R n R m is onto (or surjective) if the range of T is equal to R m (its codomain). In other words, each b in R m is the image of at least one x in R n.

6 6 DEFINITIONS AND THEOREMS Theorem. Let T : R n R m be a linear transformation with matrix A. Then the following are equivalent: T is onto T(x) = b has a solution for every b in R m Ax = b is consistent for every b in R m The columns of A span R m A has a pivot in every row. Definition. A transformation T : R n R m is one-to-one (or into, or injective) if different vectors in R n map to different vectors in R m. In other words, each b in R m is the image of at most one x in R n. Theorem. Let T : R n R m be a linear transformation with matrix A. Then the following are equivalent: T is one-to-one T(x) = b has one or zero solutions for every b in R m Ax = b has a unique solution or is inconsistent for every b in R m Ax = has a unique solution The columns of A are linearly independent A has a pivot in every column. SECTION 2.1. CHAPTER 2 Definition. The i jth entry of a matrix A is the entry in the ith row and jth column. Notation: a i j. Definition. The entries a 11, a 22, a 33,... are the diagonal entries; they form the main diagonal of the matrix. Definition. A diagonal matrix is a square matrix whose only nonzero entries are on the main diagonal. Definition. The n n identity matrix I n is the diagonal matrix with all diagonal entries equal to 1. It has the property that I n A = A for any n m matrix A. Definition. The zero matrix (of size m n) is the m n matrix with all zero entries. Definition. The transpose of an m n matrix A is the n m matrix A T whose rows are the columns of A. In other words, the i j entry of A T is a ji. Definition. The product of an m n matrix A with an n p matrix B is the m p matrix AB = where v 1, v 2,..., v p are the columns of B. Av 1 Av 2 Av p,

7 DEFINITIONS AND THEOREMS 7 Fact. Suppose A has is an m n matrix, and that the other matrices below have the right size to make multiplication work. Then: Fact. If A, B, and C are matrices, then: A(BC) = (AB)C A(B + C) = AB + AC (B + C)A = BA + CA c(ab) = (ca)b c(ab) = A(cB) I n A = A AI m = A (1) AB is usually not equal to BA. (2) AB = AC does not imply B = C. (3) AB = does not imply A = or B =. Definition. Let T : R n R m and U : R p R n be transformations. The composition is the transformation T U : R p R m defined by T U(x) = T(U(x)). Theorem. Let T : R n R m and U : R m R p be linear transformations with matrices A and B, respectively. Then the matrix for T U is AB. SECTION 2.2. Definition. A square matrix A is invertible (or nonsingular) if there is a matrix B of the same size, such that AB = I n and BA = I n. In this case we call B the inverse of A, and we write A 1 = B. Theorem. If A is invertible, then Ax = b has exactly one solution for every b, namely: x = A 1 b. Fact. Suppose that A and B are invertible n n matrices. (1) A 1 is invertible and its inverse is (A 1 ) 1 = A. (2) AB is invertible and its inverse is (AB) 1 = B 1 A 1. (3) A T is invertible and (A T ) 1 = (A 1 ) T. Theorem. Let A be an n n matrix. Here s how to compute A 1. (1) Row reduce the augmented matrix ( A I n ). (2) If the result has the form ( I n B ), then A is invertible and B = A 1. (3) Otherwise, A is not invertible. Theorem. An n n matrix A is invertible if and only if it is row equivalent to I n. In this case, the sequence of row operations taking A to I n also takes I n to A 1. Definition. The determinant of a 2 2 matrix A = a b c d is a b det(a) = det = ad bc. c d

8 8 DEFINITIONS AND THEOREMS Fact. If A is a 2 2 matrix, then A is invertible if and only if det(a). In this case, A 1 = 1 d b. det(a) c a Definition. A elementary matrix is a square matrix E which differs from the identity matrix by exactly one row operation. Fact. If E is the elementary matrix for a row operation, and A is a matrix, then EA differs from A by the same row operation. SECTION 2.3. Definition. A transformation T : R n R n is invertible if there exists another transformation U : R n R n such that T U(x) = x and U T(x) = x for all x in R n. In this case we say U is the inverse of T, and we write U = T 1. Fact. A transformation T is invertible if and only if it is both one-to-one and onto. Theorem. If T is an invertible linear transformation with matrix A, then T 1 is an invertible linear transformation with matrix A 1. I ll keep all of the conditions of the IMT right here, even though we don t encounter some until later: The Invertible Matrix Theorem. Let A be a square n n matrix, and let T : R n R n be the linear transformation T(x) = Ax. The following statements are equivalent. (1) A is invertible. (2) T is invertible. (3) A is row equivalent to I n. (4) A has n pivots. (5) Ax = has only the trivial solution. (6) The columns of A are linearly independent. (7) T is one-to-one. (8) Ax = b is consistent for all b in R n. (9) The columns of A span R n. (1) T is onto. (11) A has a left inverse (there exists B such that BA = I n ). (12) A has a right inverse (there exists B such that AB = I n ). (13) A T is invertible. (14) The columns of A form a basis for R n. (15) Col A = R n. (16) dim Col A = n. (17) rank A = n. (18) Nul A = {}. (19) dim Nul A =. (2) det(a).

9 (21) The number is not an eigenvalue of A. DEFINITIONS AND THEOREMS 9 SECTION 2.8. Definition. A subspace of R n is a subset V of R n satisfying: (1) The zero vector is in V. (2) If u and v are in V, then u + v is also in V. (3) If u is in V and c is in R, then cu is in V. Definition. If V = Span{v 1, v 2,..., v n }, we say that V is the subspace generated by or spanned by the vectors v 1, v 2,..., v n. Theorem. A subspace is a span, and a span is a subspace. Definition. The column space of a matrix A is the subspace spanned by the columns of A. It is written Col A. Definition. The null space of A is the set of all solutions of the homogeneous equation Ax = : Nul A = x Ax =. Example. The following are the most important examples of subspaces in this class (some won t appear until later): Any Span{v 1, v 2,..., v m }. The column space of a matrix: Col A = Span{columns of A}. The range of a linear transformation (same as above). The null space of a matrix: Nul A = x Ax =. The row space of a matrix: Row A = Span{rows of A}. The λ-eigenspace of a matrix, where λ is an eigenvalue. The orthogonal complement W of a subspace W. The zero subspace {}. All of R n. Definition. Let V be a subspace of R n. A basis of V is a set of vectors {v 1, v 2,..., v m } in V such that: (1) V = Span{v 1, v 2,..., v m }, and (2) {v 1, v 2,..., v m } is linearly independent. The number of vectors in a basis is the dimension of V, and is written dim V. Theorem. Every basis for a gives subspace has the same number of vectors in it. Fact. The vectors in the parametric vector form of the general solution to Ax = always form a basis for Nul A. Fact. The pivot columns of A always form a basis for Col A.

10 1 DEFINITIONS AND THEOREMS SECTION 2.9. Definition. Let B = {v 1, v 2,..., v m } be a basis of a subspace V. Any vector x in V can be written uniquely as a linear combination x = c 1 v 1 + c 2 v c m v m. The coefficients c 1, c 2,..., c m are the coordinates of x with respect to B, and the vector with entries c 1, c 2,..., c m is the B-coordinate vector of x, denoted [x] B. In summary, c 1 c [x] B = 2. means x = c 1v 1 + c 2 v c m v m. c m Definition. The rank of a matrix A, written rank A, is the dimension of the column space Col A. Rank Theorem. If A is an m n matrix, then rank A + dim Nul A = n = the number of columns of A. Basis Theorem. Let V be a subspace of dimension m. Then: Any m linearly independent vectors in V form a basis for V. Any m vectors that span V form a basis for V. Definition. The determinant is a function with the following defining properties: CHAPTER 3 det: {square matrices} R (1) If we do a row replacement on a matrix, the determinant does not change. (2) If we scale a row of a matrix by k, the determinant scales by k. (3) If we swap two rows of a matrix, the determinant scales by 1. (4) det(i n ) = 1 Magical Properties of the Determinant. (1) There is one and only one function det: {square matrices} R satisfying the defining properties (1) (4). (2) A is invertible if and only if det(a). (3) If we row reduce A to row echelon form B using r swaps, then r (product of the diagonal entries of B) det(a) = ( 1). (product of the scaling factors) (4) det(ab) = det(a) det(b) and det(a 1 ) = det(a) 1 (5) det(a) = det(a T ) (6) det(a) is the volume of the parallelepiped defined by the columns of A. (7) If A is an n n matrix with transformation T(x) = Ax, and S is a subset of R n, then the volume of T(S) is det(a) times the volume of S. (Even for curvy shapes S.)

11 DEFINITIONS AND THEOREMS 11 Definition. The i j minor of an n n matrix A is the (n 1) (n 1) matrix A i j you get by deleting the ith row and the jth column from A. Definition. The i j cofactor of A is C i j = ( 1) i+ j det A i j. Theorem. The determinant of an n n matrix A can be calculated using cofactor expansion along any row or column: n det A = a i j C i j for any fixed i det A = j=1 n a i j C i j for any fixed j i=1 Theorem. There are special formulas for determinants of 2 2 and 3 3 matrices: a b det = ad bc c d a11 a 12 a 13 det a 21 a 22 a 23 = a 11a 22 a 33 + a 12 a 23 a 31 + a 13 a 21 a 32 a a 31 a 32 a 13 a 22 a 31 a 11 a 23 a 32 a 12 a 21 a Theorem. The determinant of an upper-triangular or lower-triangular matrix is the product of the diagonal entries. Theorem. If A is an invertible n n matrix, then C 11 C 21 C 31 C n1 A 1 = 1 C 12 C 22 C 32 C n2 C det A 13 C 23 C 33 C n C 1n C 2n C 3n C nn SECTION 5.1. Definition. Let A be an n n matrix. CHAPTER 5 (1) An eigenvector of A is a nonzero vector v in R n such that Av = λv, for some λ in R. In other words, Av is a multiple of v. (2) An eigenvalue of A is a number λ in R such that the equation Av = λv has a nontrivial solution. If Av = λv for v, we say λ is the eigenvalue for v, and v is an eigenvector for λ. Fact. The eigenvalues of a triangular matrix are the diagonal entries. Fact. A matrix is invertible if and only if zero is not an eigenvalue. Fact. Eigenvectors with distinct eigenvalues are linearly independent.

12 12 DEFINITIONS AND THEOREMS Definition. Let A be an n n matrix and let λ be an eigenvalue of A. The λ-eigenspace of A is the set of all eigenvectors of A with eigenvalue λ, plus the zero vector: λ-eigenspace = v in R n Av = λv = v in R n (A λi)v = = Nul A λi. SECTION 5.2. Definition. Let A be an n n matrix. The characteristic polynomial of A is f (λ) = det(a λi). The characteristic equation of A is the equation f (λ) = det(a λi) =. Fact. If A is an n n matrix, then the characteristic polynomial of A has degree n. Fact. The roots of the characteristic polynomial (i.e., the solutions of the characteristic equation) are the eigenvalues of A. Definition. The algebraic multiplicity of an eigenvalue λ is its multiplicity as a root of the characteristic polynomial. Definition. Two n n matrices A and B are similar if there is an invertible n n matrix C such that A = CBC 1. Fact. Similar matrices have the same characteristic polynomial, hence the same eigenvalues (but different eigenvectors in general). SECTION 5.3. Definition. An n n matrix A is diagonalizable if it is similar to a diagonal matrix: A = P DP 1 for D diagonal. d 11 d Fact. If A = P DP 1 for D = , then d nn d m 11 d m A m = P D m P 1 = P P 1. d m nn

13 DEFINITIONS AND THEOREMS 13 The Diagonalization Theorem. An n n matrix A is diagonalizable if and only if A has n linearly independent eigenvectors. In this case, A = P DP 1 for λ 1 λ P = v 1 v 2 v n D = , λ n where v 1, v 2,..., v n are linearly independent eigenvectors, and λ 1, λ 2,..., λ n are the corresponding eigenvalues (in the same order). Corollary. An n n matrix with n distinct eigenvalues is diagonalizable. Procedure. How to diagonalize a matrix A: (1) Find the eigenvalues of A using the characteristic polynomial. (2) For each eigenvalue λ of A, compute a basis B λ for the λ-eigenspace. (3) If there are fewer than n total vectors in the union of all of the eigenspaces B λ, then the matrix is not diagonalizable. (4) Otherwise, the n vectors v 1, v 2,..., v n in your eigenspace bases are linearly independent, and A = P DP 1 for P = v 1 v 2 v n where λ i is the eigenvalue for v i. and D = λ 1 λ λ n Definition. Let λ be an eigenvalue of a square matrix A. The geometric multiplicity of λ is the dimension of the λ-eigenspace. Theorem. Let λ be an eigenvalue of a square matrix A. Then, 1 (the geometric multiplicity of λ) (the algebraic multiplicity of λ). Corollary. Let λ be an eigenvalue of a square matrix A. If the algebraic multiplicity of λ is 1, then the geometric multiplicity is also 1. The Diagonalization Theorem (Alternate Form). Let A be an n n matrix. The following are equivalent: (1) A is diagonalizable. (2) The sum of the geometric multiplicities of the eigenvalues of A equals n. (3) The sum of the algebraic multiplicities of the eigenvalues of A equals n, and the geometric multiplicity equals the algebraic multiplicity of each eigenvalue.

14 14 DEFINITIONS AND THEOREMS SECTION 5.5. Review. Arithmetic in the complex numbers. The Fundamental Theorem of Algebra. Every polynomial of degree n has exactly n complex roots, counted with multiplicity. Fact. Complex roots of real polynomials come in conjugate pairs. Fact. If λ is an eigenvalue of a real matrix with eigenvector v, then λ is also an eigenvalue, with eigenvector v. Theorem. Let A be a 2 2 matrix with complex (non-real) eigenvalue λ, and let v be an eigenvector. Then A = PC P 1 where P = Re v Im v Re λ and C = Im λ Im λ. Re λ The matrix C is a composition of rotation by arg(λ) and scaling by λ. Theorem. Let A be a real n n matrix. Suppose that for each (real or complex) eigenvalue, the dimension of the eigenspace equals the algebraic multiplicity. Then A = PC P 1, where P and C are as follows: (1) C is block diagonal, where the blocks are 1 1 blocks containing the real eigenvalues (with their multiplicities), or 2 2 blocks containing the matrices Re λ Im λ Im λ Re λ for each complex eigenvalue λ (with multiplicity). (2) The columns of P form bases for the eigenspaces for the real eigenvectors, or come in pairs ( Re v Im v ) for the complex eigenvectors. SECTION 6.1. CHAPTER 6 Definition. The dot product of two vectors x, y in R n is x 1 y 1 x x y = 2. y 2 def. = x 1 y 1 + x 2 y x n y n. x n y n Thinking of x, y as column vectors, this is the same as the number x T y. Definition. The length or norm of a vector x in R n is x = x x. Fact. If x is a vector and c is a scalar, then cx = c x.

15 Definition. The distance between two points x, y in R n is DEFINITIONS AND THEOREMS 15 dist(x, y) = y x. Definition. A unit vector is a vector v with length v = 1. Definition. Let x be a nonzero vector in R n. The unit vector in the direction of x is the vector x/ x. Definition. Two vectors x, y are orthogonal or perpendicular if x y =. Notation: x y. Fact. x y x y 2 = x 2 + y 2 Definition. Let W be a subspace of R n. Its orthogonal complement is Fact. Let W be a subspace of R n. (1) W is also a subspace of R n (2) (W ) = W (3) dim W + dim W = n (4) If W = Span{v 1, v 2,..., v m }, then W = v in R n v w = for all w in W. W = all vectors orthogonal to each v 1, v 2,..., v m = x in R n x v i = for all i = 1, 2,..., m v T 1 v T = Nul 2.. v T m Definition. The row space of an m n matrix A is the span of the rows of A. It is denoted Row A. Equivalently, it is the column span of A T : Row A = Col A T. It is a subspace of R n. v T 1 Fact. Span{v 1, v 2,..., v m } v T = Nul 2.. v T m Fact. Let A be a matrix. (1) (Row A) = Nul A and (Nul A) = Row A. (2) (Col A) = Nul A T and (Nul A T ) = Col A.

16 16 DEFINITIONS AND THEOREMS SECTION 6.2. Definition. Let L = Span{u} be a line in R n, and let x be in R n. The orthogonal projection of x onto L is the point proj L (x) = x u u u u. Definition. A set of nonzero vectors is orthogonal if each pair of vectors is orthogonal. It is orthonormal if, in addition, each vector is a unit vector. Lemma. A set of orthogonal vectors is linearly independent. Hence it is a basis for its span. Theorem. Let B = {u 1, u 2,..., u m } be an orthogonal set, and let x be a vector in W = Span B. Then m x u i x = u i = x u 1 u 1 + x u 2 u x u m u m. u i=1 i u i u 1 u 1 u 2 u 2 u m u m x u1 x u In other words, the B-coordinates of x are, 2 x u m,...,. u 1 u 1 u 2 u 2 u 1 u m SECTION 6.3. Definition. Let W be a subspace of R n, and let {u 1, u 2,..., u m } be an orthogonal basis for W. The orthogonal projection of a vector x onto W is m proj W (x) def x u i = u i. u i u i Fact. Let W be a subspace of R n. Every vector x can be decompsed uniquely as i=1 x = x W + x W where x W is the closest vector to x in W, and x W is in W. Theorem. Let W be a subspace of R n, and let x be a vector in R n. Then proj W (x) is the closest point to x in W. Therefore x W = proj W (x) and x W = x proj W (x). Best Approximation Theorem. Let W be a subspace of R n, and let x be a vector in R n. Then y = proj W (x) is the closest point in W to x, in the sense that dist(x, y ) dist(x, y) for all y in W. Definition. We can think of orthogonal projection as a transformation: proj W : R n R n Theorem. Let W be a subspace of R n. (1) proj W is a linear transformation. (2) For every x in W, we have proj W (x) = x. (3) For every x in W, we have proj W (x) =. (4) The range of proj W is W. x proj W (x).

17 DEFINITIONS AND THEOREMS 17 Fact. Let W be an m-dimensional subspace of R n, let proj W : R n W be the projection, and let A be the matrix for proj L. (1) A is diagonalizable with eigenvalues and 1; it is similar to the diagonal matrix with m ones and n m zeros on the diagonal. (2) A 2 = A. SECTION 6.4. The Gram Schmidt Process. Let {v 1, v 2,..., v m } be a basis for a subspace W of R n. Define: (1) u 1 = v 1 (2) u 2 = v 2 proj Span{u1 } (v 2) = v 2 v 2 u 1 u 1 u 1 u 1 (3) u 3 = v 3 proj Span{u1,u 2 } (v 3) = v 3 v 3 u 1 u 1 u 1 u 1 v 3 u 2 u 2 u 2 u 2. m 1 m. u m = v m proj Span{u1,u 2,...,u m 1 } (v m) = v m i=1 v m u i u i u i u i Then {u 1, u 2,..., u m } is an orthogonal basis for the same subspace W. QR Factorization Theorem. Let A be a matrix with linearly independent columns. Then A = QR where Q has orthonormal columns and R is upper-triangular with positive diagonal entries. Review. Procedure for computing Q and R given A. SECTION 6.5. Definition. A least squares solution to Ax = b is a vector x in R n such that for all x in R n. b A x b Ax Theorem. The least squares solutions to Ax = b are the solutions to (A T A) x = A T b. Theorem. If A has orthogonal columns v 1, v 2,..., v n, then the least squares solution to Ax = b is b v1 b v x =, 2 b v n,,. v 1 v 1 v 2 v 2 v n v n Theorem. Let A be an m n matrix. The following are equivalent: (1) Ax = b has a unique least squares solution for all b in R n. (2) The columns of A are linearly independent. (3) A T A is invertible. In this case, the least squares solution is (A T A) 1 (A T b). Review. Examples of best fit problems using least squares.

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