# The basic unit in matrix algebra is a matrix, generally expressed as: a 11 a 12. a 13 A = a 21 a 22 a 23

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1 (copyright by Scott M Lynch, February 2003) Brief Matrix Algebra Review (Soc 504) Matrix algebra is a form of mathematics that allows compact notation for, and mathematical manipulation of, high-dimensional expressions and equations For the purposes of this class, only a relatively simple exposition is required, in order to understand the notation for multivariate equations and calculations 1 Matrix Notation The basic unit in matrix algebra is a matrix, generally expressed as: a 11 a 12 a 13 A a 21 a 22 a 23 (1) a 31 a 32 a 33 Here, the matrix A is denoted as a matrix by the boldfaced type Matrices are also often denoted using bold-faced type Matrices can be of any dimension; in this example, the matrix is a 3-by-3 or 3 3 matrix The number of rows is listed first; the number of columns is listed second The subscripts of the matrix elements (a s) clarify this: the 3rd item in the second row is element a 23 A matrix with only one element (ie, 1 1 dimension) is called a scalar A matrix with only a single column is called a column vector; a matrix with only a single row is called a row vector The term vector also has meaning in analytic geometry, referring to a line segment that originates at the origin (0, 0, 0) and terminates at the coordinates listed in the k dimensions For example, you are already familiar with the Cartesian coordinate (4, 5), which is located 4 units from 0 in the x dimension and 5 units from 0 in the y dimension The vector [4, 5], then, is the line segment formed by taking a straight line from (0, 0) to (4, 5) 2 Matrix Operations The first important operation that can be performed on a matrix (or vector) is the transpose function, denoted as: A or A T The transpose function reverses the rows and columns of a matrix so that: a T ij a ji, i, j (2) This equation says that the i, j th element of the transposed matrix is the j, i th element of the original element for all i 1 I and j 1 J elements The dimensionality of a transposed matrix, therefore, is the opposite of the original matrix For example, if matrix B is 3 2, then matrix B T will be of dimension 2 3 With this basic function developed, we can now discuss other matrix functions, including matrix addition, subtraction, and multiplication (including division) Matrix addition and 1

2 subtraction are simple Provided two matrices have the same dimensionality, the addition or subtraction of two matrices proceeds by simply adding and subtracting corresponding elements in the two matrices: [ ] [ ] [ ] a11 a A + B 12 b11 b + 12 a11 + b 11 a 12 + b 12 (3) a 21 a 22 b 21 b 22 a 21 + b 21 a 22 + b 22 The commutative property of addition and subtraction that holds in scalar algebra also holds in matrix algebra: the order of addition or subtraction of matrices makes no difference to the outcome, so that A + B + C C + B + A Matrix multiplication is slightly more difficult than addition and subtraction, unless one is multiplying a matrix by a scalar In that case, the scalar is distributed to each element in the matrix, and multiplication is carried out element by element: [ ] [ ] a11 a k 12 ka11 ka 12 (4) a 21 a 22 ka 21 ka 22 In the event two matrices are being multiplied, before multiplying, one must make sure the matrices conform for multiplication This means that the number of columns in the first matrix must equal the number of rows in the second matrix For example, one can not post-multiply a 2 3 matrix A by another 2 3 matrix B, because the number of columns in A is 3, while the number of rows in B is 2 One could however multiply A by a 3 2 matrix C The matrix that results from multiplying A and C would have dimension 2 2 (same number of rows as the first matrix; same number of columns as the second matrix) The general rule for matrix multiplication is as follows: if one is multiplying A C D, then: d ij K a ik c kj, i, j (5) k1 This says that the ij th element of matrix D is equal to the sum of the multiple of the elements in row i of A and the column j of C Matrix multiplication is thus a fairly tedious process As an example, assume A is 2 3 and C is 3 2, with the following elements: A [ ], C (6) Then, element d 11 (1 1) + (2 3) + (3 5) 22, and the entire D matrix is (solve this yourself): [ ] D (7) Notice that D is 2 2 Unlike matrix addition and subtraction, in which order of the matrices is irrelevant, order matters for multiplication Obviously, given the conformability requirement, reversing the order of matrices may make multiplication impossible (eg, while a 3 2 matrix can be 2

3 post-multiplied by a 2 4 matrix, the 2 4 matrix can NOT be post-multiplied by the 3 2 matrix) However, even if matrices are conformable for multiplication after reversing their order, the resulting matrices will not generally be identical For example, a 1 k row vector multiplied by a k 1 column vector will yield a scalar (1 1), but if we reverse the order of multiplication, we will obtain a k k matrix Some additional functions that apply to matrices and are commonly seen include the trace operator (the trace of A is denoted T ra), the determinant, and the inverse The trace of a matrix is simply the sum of the diagonal elements of the matrix The determinant is more difficult Technically, the determinant is the sum of the signed multiples of all the permutations of a matrix, where permutations refer to the unique combinations of a single element from each row and column, for all rows and columns If d denotes the dimensionality of a matrix, then there are d! permutations for the matrix For instance, in a 3 3 matrix, there are a total of 6 permutations (3! ): (a 11, a 22, a 33 ), (a 12, a 23, a 31 ), (a 13, a 21, a 32 ), (a 13, a 22, a 31 ), (a 11, a 23, a 32 ), (a 12, a 21, a 33 ) Notice how for each combination, there is one element from each row and column The signing of each permutation is determined by the column position of each element in all the pairs that can be constructed using the elements of the permutation, and the subscript of element at each position in each pair For example, the permutation (a 11, a 22, a 33 ) has elements from columns 1,2, and 3 The possible ordered (i, j) pairs that can come from this permutation include (1, 2), (1, 3), and (2, 3) (based on the column position) If there are an even number of (i, j) pairs in which i > j, then the permutation is considered even and takes a positive sign; otherwise, the permutation is considered odd and takes a negative sign In this example, there are 0 pairs in which i > j, so the permutation is even (0 is even) However, in the permutation (a 13, a 22, a 31 ), the pairs are (3, 2), (3, 1), and (2, 1) In this set, all three pairs are such that i > j, hence this permutation is odd and takes a negative sign The determinant is denoted using absolute value bars on either side of the matrix name: for instance, the determinant of A is denoted as A For 2 2 and 3 3 matrices, determinants can be calculated fairly easily; however, for larger matrices, the number of permutations becomes large rapidly Fortunately, several rules simplify the process First, if any row or column in a matrix is a vector of 0, then the determinant is 0 In that case, the matrix is said not to be of full rank Second, the same is true if any two rows or columns is identical Third, for a diagonal matrix (ie, there are 0s everywhere but the main diagonal the 11, 22, 33, positions), the determinant is only the multiple of the diagonal elements There are additional rules, but they are not necessary for this brief introduction We will note that the determinant is essentially a measure of the area/volume/hypervolume bounded by the vectors of the matrix This helps, we think, to clarify why matrices with 0 vectors in them have determinant 0: just as in two dimensions a line has no area, when we have a 0 vector in a matrix, the dimensionality of the figure bounded by the matrix is reduced by a dimension (because one vector doesn t pass the origin), and hence the hypervolume is necessarily 0 Finally, a very important function for matrix algebra is the inverse function The inverse function allows the matrix equivalent of division In a sense, just as 5 times its inverse 1 1, a matrix A times its inverse denoted 5 A 1 equals I, where I is the identity matrix An identity matrix is a diagonal matrix with ones along the diagonal It is the matrix equivalent of unity (1) Some simple algebraic rules follow from the discussion of inverses 3

4 and the identity matrix: Furthermore, AA 1 A 1 A I (8) AI IA A (9) Given the commutability implicit in the above rules, it stands that inverses only exist for square matrices, and that all identity matrices are square matrices For that matter, the determinant function can only apply to square matrices also Computing the inverse of matrices is a difficult task, and there are several methods by which to derive them Probably the simplest method to compute an inverse is to use the following formula: A 1 1 adj A (10) A The only new element in this formula is the adj A, which means adjoint of A The adjoint of a matrix is the transpose of its matrix of cofactors, where a cofactor is the signed determinant of the minor of an element of a matrix The minor of element i, j can be found by deleting the ith row and jth column of the matrix For example, the minor of element a 11 of the matrix A above is: [ ] a22 a 23 (11) a 32 a 33 Taking its determinant leaves one with a scalar that is then signed (by multiplying by 1 i+j ) In this case, we obtain ( 1) 2 (a 22 a 33 a 23 a 32 ) as the cofactor for element a 11 If one replaces every element in matrix A with its signed cofactor, then transposes the result, one will obtain the adjoint of A Multiplying this by 1 (a scalar) will yield the inverse of A A There are a number of important properties of cofactors that enable more rapid computation of determinants, but discussing these is beyond the scope of this simple introduction Fortunately, computer packages tend to have determinant and inversion routines built into them, and there are plenty of inversion algorithms available if you are designing your own software, so that we generally need not worry It is worth mentioning that if a matrix has a 0 determinant, it does not have an inverse There are many additional matrix algebra rules and tricks that one may need to know; however, they are also beyond the scope of this introduction 3 The OLS Regression Solution in Matrix Form We close this section by demonstrating the utility of matrix algebra in a statistical problem: the OLS regression solution When dealing with the OLS regression problem, we can think of the entire data set in matrix terms: 4

5 y 1 y 2 y 3 y n 1 x 11 x 1k 1 x 21 x 2k 1 x 31 x 3k 1 x n1 x nk β 0 β 1 β k + In this problem, there are n individuals in the dataset measured on one dependent (outcome) variable y, with k regressor (predictor) variables x, and hence k coefficients to be estimated The column of ones represents the intercept If one performs the matrix algebra (multiplication) for the first observation, y 1, one can see that: e 1 e 2 e 3 e n (12) y 1 β 0 + β 1 x 11 + β 2 x β k x 1k + e 1 (13) which is exactly as it should be: the dependent variable for observation 1 is a linear combination of the individual s values on the regressors weighted by the regression coefficients, plus an individual-specific error term This equation can be written more succinctly by simply writing: Y Xβ + e (14) How can we solve for β? Just as in scalar algebra, we need to isolate β Unlike scalar algebra, however, we can t simply subtract the error term from both sides and divide by X, because a) there is no matrix division, really, and b) multiplication must conform So, we first multiply both sides by X : X Y X (Xβ + e) X Xβ + X e (15) We multiply by the transpose of X here, because X 1 Y would not conform for multiplication One of the assumptions of OLS regression says that X and e are uncorrelated, hence X e 0 Thus, we are left with: X Y X Xβ (16) From here, we need to eliminate X X from the right side of the equation We can do this if we take the inverse of X X and multiply both sides of the equation by it: (X X) 1 (X Y ) (X X) 1 (X X)β β (17) This follows, because A 1 A I and IA A (X X) 1 (X Y ), which should look familiar Thus, the OLS solution for β is 5

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