Big O notation
Big O notation is a symbolism used in complexity theory, computer science, and mathematics to describe the asymptotic behavior of functionss. 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 invented by the German number theorist Edmund Landau, hence it is also called Landau's symbol. The O was originally a capital omicron; today the capital letter O is used, but never the digit zero.
In Wikipedia, the various notations described in this article are used for approximating formulas (e.g. those in the sum article), for analysis of algorithms (e.g. those in the heapsort article), and for the definitions of terms in complexity theory (e.g. polynomial time).
| Table of contents |
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2 Formal definition 3 Matters of notation 4 Common orders of functions 5 Properties 6 Related asymptotic notations 7 Multiple Variables |
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.
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
constants 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 T(n) = O(n2)
and say that the algorithm has order of n2 time complexity.
Big O can also be used to describe the error term in an approximation
to a mathematical function. For instance,
Suppose f(x) and g(x) are two functions defined on
some subset of the real numbers. We say
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.
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.
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
Here is a list of classes of functions that are commonly encountered
when analyzing algorithms. All of these are for n→∞. The slower growing functions are listed
first. c is an arbitrary constant.
Uses
Infinite asymptotics
Infinitesimal asymptotics
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
if and only if
The notation can also be used to describe the behavior of f near
some real number a: we say
if and only if
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:
if and only if
Matters of notation
is to be understood as "the difference of f(x) and h(x) is O(g(x))".Common orders of functions
| 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 |
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
O(nc) 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(nc)).
The logarithms differ only by a constant factor, (since
log(nc)=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. Properties
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.
| 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)) | asymptoticaly 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) logkn) for some k. Essentially, it is Big-O, ignoring logarithmic factors. This notation is often used to describe a class of "nitpicking" estimates (since logkn is always o(n) for any constant k).