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In Einstein's special relativity, the rest energy of a mass \(m\) is \(E = mc^2\), where \(c\) is the speed of light in a vacuum.

The Taylor series for an infinitely differentiable \(f\) at point \(a\), given by \(f(x) = \sum_{k=0} ^ {\infty} \frac {f^{(k)}(a)}{k!} \, (x-a)^{k}\), allows \(f\) to be expressed in terms of its derivatives.

Factorials apply only to nonnegative integers but can be extended to complex numbers with positive real parts using the Gamma function, \(\Gamma(n) = \int_0^\infty e^{-x}x^{n-1}dx\).