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What does the formula in this book mean in mathematics?
This is the formula for calculating the correlation coefficient r of x and y.

r = cov(X,Y) / √[D(X)D(Y)]

Covariance of molecules x and y

The denominator is the standard deviation of x times the standard deviation of y.

Correlation coefficient r∈[- 1, 1]

If x and y are independent of each other, then r=0.

If r=0, then x and y are linearly independent, but not necessarily independent of each other.