Limit (mathematics)

In mathematics, the concept of a "limit" is used to describe the behavior of a function, as its argument gets "close" to either some point, or infinity; or the behavior of a sequence's elements, as their index approaches infinity. Limits are used in calculus and other branches of mathematical analysis to define derivatives and continuity.

The concept of the "limit of a function" is further generalized to the concept of topological net, while the limit of a sequence is closely related to limit and direct limit in category theory.

Limit of a function

Main article: limit of a function

Limit of a function at a point

Suppose f(x) is a real function and c is a real number. If the values of f(x) approach (get close to, but don't necessarily reach) the value L, as x approaches c, one can state that "the limit of f(x), as x approaches c, is L" and write

<math> \lim_{x \to c}f(x) = L <math>

For example: f(x)=x/(x²+1). f(1.9)=0.4121, f(1.99)=0.4012; f(1.999)=0.4001. As x approaches 2, f(x) approaches 0.4 and hence we have <math>\lim_{x\to 2}f(x)=0.4<math>.

In this case <math>\lim_{x\to 2}f(x)=f(2)<math> and f is continuous at x=2. But it is not always the case, consider

<math>g(x)=\left\{\begin{matrix} x/(x^2+1), & \mbox{if }x\ne 2 \\ 0, & \mbox{if }x=2. \end{matrix}\right.<math>

Then <math>\lim_{x\to 2}g(x)\neq g(2)<math> and so g is not continuous at x=2.

Limit of a function at infinity

One need not examine limits only as x approches some finite number; one can also examine the limit of a function as x approaches infinity. For example f(x) = 2x / x+1. f(100) = 1.9802, f(1000) = 1.9980, f(10000) = 1.9998. As x becomes extremely large, f(x) approaches 2. In this case:

<math> \lim_{x \to \infty} f(x) = 2 <math>

However, if one considers the codomain of f is the extension real line, then limit of a function at infinity could be considered as a special case of limit of a function at a point.

Limit of a sequence

Main article: limit of a sequence

Consider the following sequence: 1.79, 1.799, 1.7999,... We could observe that the numbers are "approaching" the 1.8, the limit of the sequence.

Formally, suppose x₁, x₂, ... is a sequence of real numbers. We say that the real number L is the limit of this sequence and we write

<math> \lim_{n \to \infty} x_n = L <math>

if and only if

for every ε>0 there exists a natural number n₀ (which will depend on ε) such that for all n>n₀ we have |x_n - L| < ε.

Intuitively, this means that eventually all elements of the sequence get as close as we want to the limit, since the absolute value |x_n - L| can be interpreted as the "distance" between x_n and L. Not every sequence has a limit; if it does, we call it convergent, otherwise divergent. One can show that a convergent sequence has only one limit.

The limit of a sequence and the limit of a function are closely related. On one hand, the limit of a sequence is simply the limit at infinity of a function defined on natural numbers. On the other land, a limit of a function f at x, if it exists, is the same as the limit of the sequence x_n=f(x+1/n).

Topological net

Main article: net (topology)

Better introduction is needed

All of the above notions of limit can be unified and generalized to arbitrary topological spaces by introducing topological nets and defining their limits. The article on nets elaborates on this.

An alternative is the concept of limit for filters on topological spaces.

Limit in category theory

Main article: limit (category theory)

An introduction will be added soon.