In $\mathbf X\mathbf X^\prime$, $\mathbf X$ is a matrix contains data points in column fashion, $\mathbf X^\prime$ is its transpose, this looks like a covariance matrix, but does not subtract mean, so what is the different meaning of subtract mean or not subtract it?
What is the difference between $\mathbf X\mathbf X^\prime$ and $(\mathbf X-\mu)(\mathbf X-\mu)^\prime$?
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linear-algebra
statistics
matrices