Topic: Mathematics\Calculus\Limits

Description:

In the study of calculus within the broader field of mathematics, the concept of limits serves as a foundational building block. Limits provide a rigorous way to describe the behavior of functions as inputs approach specific points, and they are indispensable in defining both derivatives and integrals.

Definition and Concept

A limit intuitively describes the value that a function approaches as the input approaches a particular point. Formally, the limit of a function $f(x)$ as $x$ approaches a value $a$ is denoted as:

$$\underset{x\to a}{lim}f(x)=L$$

This notation reads as “the limit of $f(x)$ as $x$ approaches $a$ is $L$”, and it means that as $x$ gets arbitrarily close to $a$ (but not necessarily equal to $a$), the function $f(x)$ gets arbitrarily close to $L$.

Formal Definition (ε-δ Definition)

The formal definition of a limit involves two parameters: $ϵ$ (epsilon) and $\delta$ (delta). The statement

$$\underset{x\to a}{lim}f(x)=L$$

is defined as: For every $ϵ>0$, there exists a $\delta >0$ such that if $0<|x-a|<\delta$, then $|f(x)-L|<ϵ$.

This formalism ensures that the values of $f(x)$ can be made as close as desired to $L$ by choosing $x$ sufficiently close to $a$.

One-Sided Limits

Sometimes, it is useful to consider limits from only one side of the point $a$. These are called one-sided limits:

• Right-hand limit: The limit of $f(x)$ as $x$ approaches $a$ from the right (values greater than $a$) is denoted by

$$\underset{x\to a^{+}}{lim}f(x)=L$$

• Left-hand limit: The limit of $f(x)$ as $x$ approaches $a$ from the left (values less than $a$) is denoted by

$$\underset{x\to a^{-}}{lim}f(x)=L$$

A limit exists only if both one-sided limits exist and are equal:

$$\underset{x\to a}{lim}f(x)=L⟺\underset{x\to a^{+}}{lim}f(x)=\underset{x\to a^{-}}{lim}f(x)=L$$

Infinite Limits and Limits at Infinity

Limits can also involve scenarios where $x$ approaches infinity, or the function values $f(x)$ approach infinity. These are handled with the following notations:

• As $x$ approaches infinity:

$$\underset{x\to \mathrm{\infty }}{lim}f(x)=L$$

• As the function values become unbounded:

$$\underset{x\to a}{lim}f(x)=\mathrm{\infty }$$

These concepts are crucial for understanding the behavior of functions over large domains or near singularities.

Applications of Limits

1. Continuity: A function $f$ is continuous at a point $a$ if

$$\underset{x\to a}{lim}f(x)=f(a)$$

2. Derivatives: The derivative of a function at a point $a$ is defined using a limit:

$$f^{\prime }(a)=\underset{h\to 0}{lim}\frac{f(a+h)-f(a)}{h}$$

3. Integrals: The definite integral of a function over an interval $[a,b]$ is defined using sums that approach the exact area under the curve as limits:

$${\int }_a^{b}f(x)dx=\underset{n\to \mathrm{\infty }}{lim}\sum _{i=1}^{n}f(x_i^{\ast })\mathrm{\Delta }x$$

Conclusion

Understanding limits is crucial for mastering calculus, as they underpin many advanced concepts and techniques. By precisely defining how functions behave near points of interest, limits enable the rigorous treatment of continuity, differentiation, and integration, which are essential tools in mathematical analysis and numerous applications in science and engineering.