Big O notation

Big O notation is a type of symbolism used in complexity theory, computer science, and mathematics to describe the asymptotic behavior of functions. More exactly, it is used to describe an asymptotic upper bound for the magnitude of a function in terms of another, usually simpler, function.

It was first introduced by German number theorist Paul Bachmann in his 1892 book Analytische Zahlentheorie. The notation was popularized in the work of another German number theorist Edmund Landau, hence it is sometimes called a Landau symbol. The O was originally a capital omicron; today the capital letter O is used, but never the digit zero.

Omega and theta notation are also used for approximating formulae (see sum), analyzing algorithms (see heapsort), and for defining terms in complexity theory (see polynomial time).

Uses

There are two formally close, but noticeably different usages of this notation: infinite asymptotics and infinitesimal asymptotics. This distinction is only in application and not in principle, however—the formal definition for the "big O" is the same for both cases, only with different limits for the function argument.

Infinite asymptotics

Big O notation is useful when analyzing algorithms for efficiency. For example, the time (or the number of steps) it takes to complete a problem of size n might be found to be T(n) = 4n² - 2n + 2.

As n grows large, the n² term will come to dominate, so that all other terms can be neglected. Further, the coefficients will depend on the precise details of the implementation and the hardware it runs on, so they should also be neglected. Big O notation captures what remains: we write

<math>T(n)\in O(n^2)<math>

and say that the algorithm has order of n² time complexity.

Infinitesimal asymptotics

Big O can also be used to describe the error term in an approximation to a mathematical function. For example,

<math>e^x=1+x+x^2/2+\hbox{O}(x^3)\qquad\hbox{as}\ x\to 0<math>

expresses the fact that the error is smaller in absolute value than some constant times x³ if x is close enough to 0.

Formal definition

Suppose f(x) and g(x) are two functions defined on some subset of the real numbers. We say

f(x) is O(g(x)) as x → ∞

if and only if

there exists numbers x₀ and M such that |f(x)| ≤ M |g(x)| for x > x₀.

The notation can also be used to describe the behavior of f near some real number a: we say

f(x) is O(g(x)) as x → a

if and only if

there exists numbers δ>0 and M such that |f(x)| ≤ M |g(x)| for |x - x₀| < δ.

If g(x) is non-zero for values of x sufficiently close to a, both of these definitions can be unified using the limit superior:

f(x) is O(g(x)) as x → a

if and only if

<math>\limsup_{x\to a} \left|\frac{f(x)}{g(x)}\right| < \infty<math>

In mathematics, both asymptotic behaviors near ∞ and near a are considered. In computational complexity theory, only asymptotics near ∞ are used; furthermore, only positive functions are considered, so the absolute value bars may be left out.

Matters of notation

The statement "f(x) is O(g(x))" as defined above is often written as f(x) = O(g(x)). This is a slight abuse of notation: we are not really asserting the equality of two functions. Here is a bad example:

<math>O(x)=O(x^2)\ \ \mbox{but}\ \ O(x^2)\, \not=\, O(x)<math>

By this reason, some authors prefer a set notation and write f ∈ O(g), thinking of O(g) as the set of all functions dominated by g.

Furthermore, an "equation" of the form

f(x) = h(x) + O(g(x))

is to be understood as "the difference of f(x) and h(x) is O(g(x))".

Common orders of functions

Here is a list of classes of functions that are commonly encountered when analyzing algorithms. All of these are as n increases to infinity. The slower-growing functions are listed first. c is an arbitrary constant.

notation	name
O(1)	constant
O(log n)	logarithmic
O([log n]c)	polylogarithmic
O(n)	linear
O(n · log n)	sometimes called "linearithmic"
O(n2)	quadratic
O(nc)	polynomial, sometimes "geometric"
O(cn)	exponential
O(n!)	factorial
O(nn)	?

Properties

If a function f(n) can be written as a finite sum of other functions, then the fastest growing one determines the order of f(n). For example

<math>f(n) = 10 \log n + 5 (\log n)^3 + 7n + 3n^2 + 6n^3 = \hbox{O}(n^3)\,\!<math>.

In particular, if a function may be bounded by a polynomial in n, then as n tends to infinity, one may disregard lower-order terms of the polynomial.

O(n^c) and O(cⁿ) are very different. The latter grows much, much faster, no matter how big the constant c is. A function that grows faster than any power of n is called superpolynomial. One that grows slower than an exponential function of the form cⁿ is called subexponential. An algorithm can require time that is both superpolynomial and subexponential; examples of this include the fastest algorithms known for integer factorization.

O(log n) is exactly the same as O(log(n^c)). The logarithms differ only by a constant factor, (since log(n^c)=c log n) and thus the big O notation ignores that. Similarly, logs with different constant bases are equivalent.

If a function f(x) may be bounded by a polynomial in x, then as x tends to zero, one may disregard higher-order terms of the polynomial. Notice the distinction with the case of infinite asymptotics.

Related asymptotic notations: O, o, Ω, ω, Θ, Õ

Big O is the most commonly used asymptotic notation for comparing functions. We will define some others briefly in terms of "big O":

Notation	Definition
f(n) = O(g(n))	asymptotic upper bound
f(n) = o(g(n))	asymptotically negligible (M = 0)
f(n) = Ω(g(n))	asymptotic lower bound (iff g(n) = O(f(n)))
f(n) = ω(g(n))	asymptotically dominant (iff g(n) = o(f(n)))
f(n) = Θ(g(n))	asymptotically tight bound (iff both f(n) = O(g(n)) and g(n) = O(f(n)))

Here is a hint (and mnemonics) why Landau selected these Greek letters: "omicron" is "o-micron", i.e., "o-small", whereas "omega" is "o-BIG".

The relation f(n) = o(g(n)) is read as "f(n) is little-oh of g(n)". Intuitively, it means that g(n) grows much faster than f(n). Formally, it states that the limit of f(n)/g(n) is zero.

The notations Θ and Ω are often used in computer science; the lower-case o is common in mathematics but rare in computer science. The lower-case ω is rarely used.

In casual use, O is commonly used where Θ is meant, i.e., a tight estimate is implied. For example, one might say "heapsort is O(n log n) in average case" when the intended meaning was "heapsort is Θ(n log n) in average case". Both statements are true, but the latter is a stronger claim.

Another notation sometimes used in computer science is Õ (read Soft-O). f(n) = Õ(g(n)) is shorthand for f(n) = O(g(n) log^kn) for some k. Essentially, it is Big-O, ignoring logarithmic factors. This notation is often used to describe a class of "nitpicking" estimates (since log^kn is always o(n) for any constant k).

Multiple variables

Big O (and little o, and Ω...) can also be used with multiple variables. For example, the statement

<math>f(n,m) = n^2 + m^3 + \hbox{O}(n+m) \mbox{ as } n,m\to\infty<math>

asserts that there exist constants C and N such that

<math>\forall n, m>N:f(n,m) \le n^2 + m^3 + C(n+m)<math>

To avoid ambiguity, the running variable should always be specified: the statements

<math>f(n,m) = \hbox{O}(n^m) \mbox{ as } n,m\to\infty<math>

is quite different from

<math>\forall m: f(n,m) = \hbox{O}(n^m) \mbox{ as } n\to\infty<math>

C