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Unformatted text preview: MA 265 LECTURE NOTES: FRIDAY, JANUARY 18 Special Types of Matrices In todays lecture, we only consider n n matrices. Square Matrices. We give a few definitions for n n matrices: Let A = [ a ij ] be an n n matrix i.e. a matrix which has the same number of rows as columns. Such a matrix is called a square matrix . Recall that the elements a 11 , a 22 , ..., a nn compose the main diagonal of A . We say A is upper triangular if a ij = 0 for i > j : A = a 11 a 12 a 1 n a 22 a 2 n . . . . . . . . . . . . a nn . Similarly, we say A is lower triangular if a ij = 0 for i < j . Equivalently, A is lower triangular when its transpose A T is upper triangular. We say A is a diagonal matrix if the entries off the main diagonal are zero i.e. a ij = 0 for i negationslash = j . Equivalently, a diagonal matrix is a matrix that is both upper triangular and lower triangular. We say A is a scalar matrix if there is a fixed number r such that a ij = r for i = j yet a ij = 0 for i negationslash = j : A = r r . ....
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 Spring '10
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