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Such spectra show kinetic energies of individual electrons but the correlations between the electrons are lost in the process of averaging. To reveal the correlations we need to calculate the covariance map:
:<math>\mathbf{cov}(\mathbf{Y},\mathbf{X}) = \langle \mathbf{YX} \rangle - \langle \mathbf{Y} \rangle \langle \mathbf{X} \rangle </math>,
where vector <math>\mathbf{Y}</math> is the [[transpose]] of vector <math>\mathbf{X}</math> and the angular brackets denote averaging over many laser shots as before. Note that the ordering of the vectors (a column followed by a row) ensures that [[Matrix multiplication|their multiplication]] gives a matrix. It is convenient to display the matrix as a false-colour map.
===How to read the map===
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