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You might like to note that the sample correlation r_{xy} of a pair of lists of N numbers x_i, y_i is given by
r_{xy} = \frac{\sum^{N}_{i=1} \left((x_i-\bar{x})(y_i-\bar{y})\right)}{\sqrt{\sum^N_{i=1}(x-\bar{x})^2\sum^{N}_{i=1}(y_i-\bar{y})^2}}
Explore the properties of this different sort of differential equation.