weatherwax_Box_N_Jenkins - Notes on the Book: Time Series...

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Unformatted text preview: Notes on the Book: Time Series Analysis: Forecasting and Control by George E. P. Box and Gwilym M. Jenkins John L. Weatherwax ∗ June 20, 2008 * [email protected] 1 Chapter 2 (The Autocorrelation Function and the Spec- trum) Notes on the Text Notes on positive definiteness and the autocovariance matrix The book defined the autocovariance matrix Γ n of a stochastic process as Γ n = γ γ 1 γ 2 ··· γ n − 1 γ 1 γ γ 1 ··· γ n − 2 γ 2 γ 1 γ ··· γ n − 3 . . . . . . . . . ··· . . . γ n − 1 γ n − 2 γ n − 3 ··· γ . (1) Then holding the definition for a second, if we consider the derived time series L t given by L t = l 1 z t + l 2 z t − 1 + ··· + l n z t − n +1 , we can compute the variance of this series using the definition var[ L t ] = E [( L t − ¯ L ) 2 ]. We first evaluate the mean of L t ¯ L = E [ l 1 z t + l 2 z t − 1 + ··· + l n z t − n +1 ] = ( l 1 + l 2 + ··· + l n ) μ , since z t is assumed stationary so that E [ z t ] = μ for all t . We then have that L t − ¯ L = l 1 ( z t − μ ) + l 2 ( z t − 1 − μ ) + l 3 ( z t − 2 − μ ) + ··· + l n ( z t − n +1 − μ ) , so that when we square this expression we get ( L t − ¯ L ) 2 = n summationdisplay i =1 n summationdisplay j =1 l i l j ( z t − ( i − 1) − μ )( z t − ( j − 1) − μ ) . Taking the expectation of both sides to compute the variance and using E [( z t − ( i − 1) − μ )( z t − ( j − 1) − μ )] = γ | i − j | , gives var[ L t ] = n summationdisplay i =1 n summationdisplay j =1 l i l j γ | i − j | . As the expression on the right-hand-side is the same as the quadratic form bracketleftbig l 1 l 2 l 3 ··· l n bracketrightbig γ γ 1 γ 2 ··· γ n − 1 γ 1 γ γ 1 ··· γ n − 2 γ 2 γ 1 γ ··· γ n − 3 . . . . . . . . . ··· . . . γ n − 1 γ n − 2 γ n − 3 ··· γ l 1 l 2 l 3 . . . l n . (2) 2 Thus since var[ L t ] > 0 (from its definition) for all possible values for l 1 ,l 2 ,l 3 , ··· l n − 1 we have shown that the inner product given by Equation 2 is positive for all nonzero vectors with components l 1 ,l 2 ,l 3 , ··· l n − 1 we have shown that the autocovariance matrix Γ n is positive definite. Since the autocorrelation matrix, P n , is a scaled version of Γ n it too is positive definite. Given the fact that P n is positive definite we can use standard properties of positive definite matrices to derive properties of the correlations ρ k . Given a matrix Q of size n × n , we define the principal minors of Q to be determinants of smaller square matrices obtained from the matrix Q . The smaller submatrices are selected from Q by selecting a set of indices from 1 to n representing the rows (and columns) we want to downsample from. Thus if you view the indices selected as the indices of rows from the original matrix...
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This note was uploaded on 04/01/2012 for the course ORIE 5550 taught by Professor Matteson during the Spring '12 term at Cornell University (Engineering School).

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weatherwax_Box_N_Jenkins - Notes on the Book: Time Series...

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