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Shannon's entropy, \(H(X) = -\sum_{x \in \mathbf{X}} p(x) \log p(x)\), measures how informative the mass function \(p\) is about the occurrence of the possible values of variable \(X\).

The fundamental theorem of calculus, \(f(x) = \frac{d}{dx}\int_{a}^{x}f(s)ds\), expresses the essential inverse relationship between the derivative and integral operators.

Integrating the Gaussian function from minus to plus infinity yields \(\int_{-\infty}^{\infty} e^{-x^2}dx=\sqrt{\pi}\), which is known as the Gaussian integral.