Logistic function

Definition

The logistic function is a function with domain $\mathbb {R}$ and range the open interval $(0,1)$, defined as:

$x\mapsto {\frac {1}{1+e^{-x}}}$

Equivalently, it can be written as:

$x\mapsto {\frac {e^{x}}{e^{x}+1}}$

Yet another form that is sometimes used, because it makes some aspects of the symmetry more evident, is:

$x\mapsto {\frac {e^{x/2}}{e^{x/2}+e^{-x/2}}}$

For this page, we will denote the function by the letter $g$.

We may extend the logistic function to a function $[-\infty ,\infty ]\to [0,1]$, where $g(-\infty )=0$ and $g(\infty )=1$.

Probabilistic interpretation

The logistic function transforms the logarithm of the odds to the actual probability. Explicitly, given a probability $p$ (strictly between 0 and 1)of an event occurring, the odds in favor of $p$ are given as:

${\frac {p}{1-p}}$

This could take any value in $(0,\infty )$

The logarithm of odds is the expression:

$\ln \left({\frac {p}{1-p}}\right)$

If $x$ equals the above expression, then the function describing $p$ in terms of $x$ is the logistic function.

Key data

Item	Value
default domain	all of $\mathbb {R}$, i.e., all reals
range	the open interval $(0,1)$, i.e., the set $\{x\mid 0\leq x\leq 1\}$
derivative	the derivative is ${\frac {-e^{-x}}{(1+e^{-x})^{2}}}$. If we denote the logistic function by the letter $g$, then we can also write the derivative as $g'(x)=g(x)g(-x)=g(x)(1-g(x))$
second derivative	If we denote the logistic function by the letter $g$, then we can also write the derivative as $g'(x)=g(x)g(-x)=g(x)(1-g(x))(1-2g(x))$
logarithmic derivative	the logarithmic derivative is ${\frac {e^{-x}}{1+e^{-x}}}$ If we denote the logistic function by $g$, the logarithmic derivative is $g(-x)$
antiderivative	the function $x\mapsto \ln(e^{x}+1)+C=-\ln(g(-x))+C$
critical points	none
critical points for the derivative (correspond to points of inflection for the function)	$x=0$; the corresponding point on the graph of the function is $(0,1/2)$.
local maximal values and points of attainment	none
local minimum values and points of attainment	none
intervals of interest	increasing and concave up on $(-\infty ,0)$ increasing and concave down on $(0,\infty )$
horizontal asymptotes	asymptote at $y=0$ corresponding to the limit for $x\to -\infty$ asymptote at $y=1$ corresponding to the limit for $x\to \infty$
inverse function	inverse logistic function or log-odds function given by $x\mapsto \ln \left({\frac {x}{1-x}}\right)$

Differentiation

First derivative

Consider the expression for $g(x)$:

$g(x)={\frac {1}{1+e^{-x}}}=(1+e^{-x})^{-1}$

We can differentiate this using the chain rule for differentiation (the inner function being $x\mapsto 1+e^{-x}$ and the outer function being the reciprocal function $t\mapsto 1/t$. We get:

$g'(x)=-(1+e^{-x})^{-2}(-e^{-x})$

Simplifying, we get:

$g'(x)={\frac {e^{-x}}{(1+e^{-x})^{2}}}$

We can write this in an alternate way that is sometimes more useful. We split the expression as a product:

$g'(x)=\left({\frac {1}{1+e^{-x}}}\right)\left({\frac {e^{-x}}{1+e^{-x}}}\right)$

The first factor on the right is $g(x)$, and the second factor is $1-g(x)$, so this simplifies to:

$g'(x)=g(x)(1-g(x))$

Functional equations

Symmetry equation

The logistic function $g$ has the property that its graph $y=g(x)$ has symmetry about the point $(0,1/2)$. Explicitly, it satisfies the functional equation:

$g(x)+g(-x)=1$

We can see this algebraically:

$g(-x)={\frac {1}{1+e^{-(-x)}}}={\frac {1}{1+e^{x}}}$

Multiply numerator and denominator by $e^{-x}$, and get:

$g(-x)={\frac {e^{-x}}{e^{-x}+1}}=1-{\frac {1}{1+e^{-x}}}=1-g(x)$

Differential equation

As discussed in the #First derivative section, the logistic function satisfies the condition:

$g'(x)=g(x)(1-g(x))$

Therefore, $y=g(x)$ is a solution to the autonomous differential equation:

${\frac {dy}{dx}}=y(1-y)$

The general solution to that equation is the function $y=g(x+C)$ where $C\in \mathbb {R}$. The initial condition $y=1/2$ at $x=0$ pinpoints the logistic function uniquely.

Integration

First antiderivative

Direct computation

We have:

$\int g(x)\,dx=\int {\frac {1\,dx}{1+e^{-x}}}=\int {\frac {e^{x}\,dx}{e^{x}+1}}=\ln(e^{x}+1)+C$

Computation in terms of functional equations for the logistic function

We have:

$g'(x)=g(x)(1-g(x))$

We also have that $1-g(x)=g(-x)$, so we get:

$g'(x)=g(x)g(-x)$

This can be rewritten as:

${\frac {d}{dx}}(\ln(g(x))=g(-x)$

By the chain rule for differentiation, we get:

${\frac {d}{dx}}(\ln(g(-x))=-g(x)$

Thus:

$\int g(x)\,dx=-\ln(g(-x))+C$

This can be simplified and verified to be the same as the answer obtained by direct computation.