Error incurred upon truncating a diagonal matrix to only the k largest eigenvalues.

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Say I have a diagonal matrix (A) of dim n x n. What is the error incurred if I approximate it with only its k-largest eigenvalues? I am using the 1-norm. I am trying to quantify the error in the following manner: Err=||A||-||A1||, where ||A|| is the original diagonal matrix and ||A1|| is the truncated diagonal matrix.