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The exponential function to the natural base, \(e^x\), can be defined in more than one way. One of them makes use of the integral \(\int_1^{e^x} \frac{dy}{y}= x\).

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

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.