Sigmoid function

A sigmoid function is a mathematical function having an "S" shaped curve (sigmoid curve). Often, sigmoid function refers to the special case of the logistic function shown in the first figure and defined by the formula

Other examples of similar shapes include the Gompertz curve (used in modeling systems that saturate at large values of t) and the ogee curve (used in the spillway of some dams). Sigmoid functions have finite limits at negative infinity and infinity, most often going either from 0 to 1 or from −1 to 1, depending on convention.

A wide variety of sigmoid functions have been used as the activation function of artificial neurons, including the logistic and hyperbolic tangent functions. Sigmoid curves are also common in statistics as cumulative distribution functions (which go from 0 to 1), such as the integrals of the logistic distribution, the normal distribution, and Student's t probability density functions.

A sigmoid function is a bounded differentiable real function that is defined for all real input values and has a positive derivative at each point.

In general, a sigmoid function is real-valued and differentiable, having either a non-negative or non-positive first derivative which is bell shaped. There are also a pair of horizontal asymptotes as ${\displaystyle t\rightarrow \pm \infty }$. The differential equation ${\displaystyle {\tfrac {\mathrm {d} }{\mathrm {d} t}}S(t)=c_{1}S(t)\left(c_{2}-S(t)\right)}$, with the inclusion of a boundary condition providing a third degree of freedom, ${\displaystyle c_{3}}$, provides a class of functions of this type.

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