# Definition:O Notation/Big-O Notation

## Definition

**Big-O notation** occurs in a variety of contexts.

### Sequences

Let $\sequence {a_n}$ and $\sequence {b_n}$ be sequences of real or complex numbers.

**$a_n$ is big-O of $b_n$** if and only if

- $\exists c \in \R: c \ge 0 : \exists n_0 \in \N : \paren {n \ge n_0 \implies \size {a_n} \le c \cdot \size {b_n} }$

That is:

- $\size {a_n} \le c \cdot \size {b_n}$

for all sufficiently large $n$.

### Real Analysis

Let $f$ and $g$ be real-valued or complex-valued functions defined on a neighborhood of $+ \infty$ in $\R$.

The statement:

- $\map f x = \map {\mathcal O} {\map g x}$ as $x \to \infty$

is equivalent to:

- $\exists c \in \R: c \ge 0: \exists x_0 \in \R: \forall x \in \R: \paren {x \ge x_0 \implies \size {\map f x} \le c \cdot \size {\map g x} }$

That is:

- $\size {\map f x} \le c \cdot \size {\map g x}$

for $x$ sufficiently large.

This statement is voiced **$f$ is big-O of $g$** or simply **$f$ is big-O $g$**.

### Complex Analysis

Let $f$ and $g$ be complex functions defined for all complex numbers whose modulus is sufficiently large.

The statement:

- $f(z) = \mathcal O \left({g(z)}\right)$ as $|z|\to\infty$

is equivalent to:

- $\displaystyle \exists c\in \R: c\ge 0 : \exists r_0 \in \R : \forall z \in \C : (|z| \geq r_0 \implies |f(z)| \leq c \cdot |g(z)|)$

That is:

- $|f(z)| \leq c \cdot |g(z)|$

for all $z$ in a neighborhood of infinity in $\CC$.

### General Definition

Let $\struct {X, \tau}$ be a topological space.

Let $V$ be a normed vector space over $\R$ or $\C$ with norm $\norm {\,\cdot\,}$.

Let $f, g : X \to V$ be functions.

The statement:

- $\map f x = \map \OO {\map g x}$ as $x \to \infty$

is equivalent to:

- There exists a neighborhood of infinity $U \subset X$ such that:
- $\exists c \in {\R}_{\ge 0}: \forall x \in U: \norm {\map f x} \le c \norm {\map g x}$

That is:

- $\norm {\map f x} \le c \norm {\map g x}$

for all $x$ in a neighborhood of infinity.

## Dependence on Parameters

Let $X$ be a topological space.

Let $V$ be a normed vector space over $\R$ or $\C$ with norm $\norm {\, \cdot \,}$

Let $x_0 \in X \cup \set \infty$.

Let $A$ be a set.

Let $X_\alpha$ be a subset of $X$ for every $\alpha \in A$, which, if $x_0 \ne \infty$, contains $x_0$.

Let $f_\alpha : X_\alpha \setminus \set {x_0} \to V$ be a mapping for every $\alpha \in A$.

Let $g : X \to V$ be a mapping.

The statement:

- $f_\alpha = \map {\OO_\alpha} g$ as $x \to x_0$

is equivalent to:

- $\forall \alpha \in A : f_\alpha = \map \OO g$ as $x \to x_0$

The $\OO$-estimate is said to be **independent** of $\alpha \in A$ if and only if:

- there exists a neighborhood $U$ of $x_0$ in $X$ such that:
- $\exists c \in \R: c \ge 0 : \forall \alpha \in A : \forall x \in \paren {U \setminus \set {x_0} } \cap X_\alpha : \norm {\map {f_\alpha} x} \le c \cdot \norm {\map g x}$

That is, if the implied constant and implied neighborhood can be chosen the same for all $\alpha \in A$.

## Uniform Estimates

Let $X$ be a set.

Let $V$ be a normed vector space over $\R$ or $\C$ with norm $\norm {\, \cdot \,}$.

Let $f, g : X \to V$ be mappings.

Then **$f$ is big $\OO$ of $g$ uniformly** if and only if:

- $\exists c > 0 : \forall x \in X : \norm {\map f x} \le c \cdot \norm {\map g x}$

This is denoted:

- $f = \map \OO g$

## Implied Constant

From the definition of the limit of a function, it can be seen that this is also equivalent to:

- $\exists c \in \R: c > 0, k \ge 0: \forall n > k, f \left({n}\right) \le c g \left({n}\right)$

For some fixed $k$ (appropriate to the function under consideration) the infimum of such $c$ is called the **implied constant**.

## Also known as

The big-$\OO$ notation, along with little-$\mathcal o$ notation, are also referred to as **Landau's symbols** or **the Landau symbols**, for Edmund Georg Hermann Landau.

In analytic number theory, sometimes **Vinogradov's notations** $f \ll g$ or $g \gg f$ are used to mean $f = \map \OO g$.

This can often be clearer for estimates leading to typographically complex error terms.

Some sources use an ordinary $O$:

- $f = \map O g$

## Also defined as

Some authors require that the inequality be valid on the entire domain of definition.

On $\mathsf{Pr} \infty \mathsf{fWiki}$, this is known as a uniform big-$\OO$ estimate.

The statement $f = \map \OO g$ is sometimes seen to be defined as:

- $\displaystyle \exists \alpha \in \R: \alpha \ge 0 : \lim_{x \mathop \to \infty} \frac {\map f x} {\map g x} = \alpha$

However, requiring that the limit exists is generally viewed to be too restrictive.

## Also see

- Results about
**asymptotic notations**can be found here.

## Sources

- 1932: A.E. Ingham:
*The Distribution of Prime Numbers*: $\S 1.1$ - 1994: H.E. Rose:
*A Course in Number Theory*(2nd ed.) ... (previous) ... (next): Preface to first edition: Prerequisites - 2008: David Nelson:
*The Penguin Dictionary of Mathematics*(4th ed.) ... (previous) ... (next): Entry:**order notation**