Let A be an m ×n matrix with full column rank. Assume that the Q R factorization of A is A=QR0 where Q is orthogonal and R is upper triangular. An important matrix in statistical applications is the variance-covariance matrix C : C=(A^T A)^-1 (a) Give an expression for the matrix C in terms of matrices from the Q R factorization of A. (b) Explain how to compute the explicit elements of C using the Q R factorization of A. How many flops are required for your procedure? (c) Explain how to compute

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