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Linearization in mathematics and its applications in general refers to finding the linear approximation to a function at a given point. In the study of dynamical systems, linearization is a method for assessing the local stability of an equilibrium point of a system of nonlinear differential equations. This method is used in fields such as engineering, physics, economics, and ecology.

Linearizations of a function are lines; ones that are usually used for purposes of calculation. Linearization is an efficacious method used to approximate the output of function at any x = a based on the value and slope of a function y = f(x) at x = b, given that f(x) is continuous on [a,b] (or [b,a])and that a is close to b. In, short, linearization approximates the output of a function near x = a.

For example, you might know that . But without a calculator, what would be a good approximation of ?

For any given function y = f(x), f(x) can be approximated if it is near a known continuous point. The most basic requisite is that, where L_a(x) is the linearization of f(x) at x = a, L(a) = f(a). The point-slope form of an equation forms an equation of a line, given a point (H,K) and slope M. The general form of this equation is: y − K = M(x − H).

Using the point (x,f(x)), L_a(x) becomes y = f(a) + M(x − a). Because continuous functions are locally linear, the best slope to substitute in would be the slope of the line tangent to f(x) at x = a.

While the concept of local linearity applies the most to points arbitrarily close to x = a, those relatively close work relatively well for linear approximations. After all, a linearization is only an approximation. The slope M should be, most accurately, the slope of the tangent line at x = a.

Visually, the accompanying diagram shows the tangent line of f(x) at x. At f(x + h), where h is any small positive or negative value, f(x+h) is very nearly the value of the tangent line at the point (x + h,L(x + h)).

The final equation for the linearization of a function at x = a is:

For x = a, f(a) is f(x) at a. The derivative of f(x) is f'(x), and the slope of f(x) at a is f'(a).

Instead of a linear function, perhaps, would x not be better approximated with polynomial? This can be accomplished via Taylor series. This is not the purpose of linearization though.

To find , we can use the fact that . The linearization of at x = a is , because the function defines the slope of the function at x. Plugging in a = 4, the linearization at 4 is . In this case x = 4.001, so is approximately . The true value is closer to 2.00024998, which is amazingly accurate.

The analysis of linear functions is well defined, but most representations of actual systems are nonlinear. Linearization allows us to use tools for studying linear systems to analyze the behavior of a nonlinear function near a given point. The linearization of a function is the first order term of its Taylor expansion around the point of interest. For a system defined by the equation

,

the linearized system can be written as

where is the point of interest and is the Jacobian of evaluated at .

In stability analysis, one can use the eigenvalues of the Jacobian matrix evaluated at an equilibrium point to determine the nature of that equilibrium. If all the eigenvalues are positive, the equilibrium is unstable; if they are all negative the equilibrium is stable; and if the values are of mixed signs, the equilibrium is a saddle point. Any complex eigenvalues will appear in complex conjugate pairs and indicate spiral (or circular if the real components are zero) around the equilibrium.