Is there any way to estimate the covariance matrix of a nonlinear function?

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Suppose $\mathbf{x}=f(\mathbf{y})$ where vectors $\mathbf{x}$ and $\mathbf{y}$ $\in R^N$, and the covariance matrix of $\mathbf{y}$ is given as $\sigma_y$. How do we estimate the covariance matrix of $\mathbf{x}$?