# Almost all

In mathematics, the term "**almost all**" means "all but a negligible amount". More precisely, if `X` is a set, "almost all elements of `X`" means "all elements of `X` but those in a negligible subset of `X`". The meaning of "negligible" depends on the mathematical context; for instance, it can mean finite, countable, or null.

In contrast, "**almost no**" means "a negligible amount"; that is, "almost no elements of `X`" means "the elements of some negligible subset of `X`".

## Contents

## Meanings in areas of mathematicsEdit

### Prevalent meaningEdit

Throughout mathematics, "almost all" is sometimes used to mean "all (elements of an infinite set) but finitely many".^{[1]}^{[2]}^{[3]} This use occurs in philosophy as well.^{[4]} Similarly, "almost all" can mean "all (elements of an uncountable set) but countably many".^{[sec 1]}

Examples:

- Almost all positive integers are greater than 1,000,000,000,000.
^{[5]}^{:293} - Almost all prime numbers are odd, as 2 is the only exception.
- Almost all polyhedra are irregular, as there are only nine exceptions: the five platonic solids and the four Kepler-Poinsot polyhedra.
- If
`P`is a nonzero polynomial,`P(x)`≠0 for almost all`x`.

### Meaning in measure theoryEdit

When speaking about the reals, sometimes "almost all" means "all reals but a null set".^{[6]}^{[7]}^{[sec 2]} Similarly, if `S` is some set of reals, "almost all numbers in `S`" can mean "all numbers in `S` but those in a null set".^{[8]} The real line can be thought of as a one-dimensional Euclidean space. In the more general case of an `n`-dimensional space (where `n` is a positive integer), these definitions can be generalised to "all points but those in a null set"^{[sec 3]} or "all points in `S` but those in a null set" (this time, `S` is a set of points in the space).^{[9]} Even more generally, "almost all" is sometimes used in the sense of "almost everywhere" in measure theory,^{[10]}^{[11]}^{[sec 4]} or in the closely related sense of "almost surely" in probability theory.^{[11]}^{[sec 5]}

Examples:

- In a measure space, such as the real line, countable sets are null. The set of rational numbers is countable, and thus almost all real numbers are irrational.
^{[12]} - As Georg Cantor proved in his first set theory article, the set of algebraic numbers is countable as well, so almost all reals are transcendental.
^{[13]} - Almost all reals are normal.
^{[14]} - The Cantor set is null as well. Thus, almost all reals are not members of it even though it is uncountable.
^{[6]} - The derivative of the Cantor function is 0 for almost all numbers in the unit interval.
^{[15]}It follows from the previous example because the Cantor function is locally constant—and thus with derivative 0—outside the Cantor set.

### Meaning in number theoryEdit

In number theory, "almost all positive integers" can mean "the positive integers in a set whose natural density is 1". That is, if `A` is a set of positive integers, and if the proportion of positive integers below `n` that are in `A` (out of all positive integers below `n`) tends to 1 as `n` tends to infinity, then almost all positive integers are in `A`.^{[16]}^{[17]}^{[sec 6]} More generally, let `S` be an infinite set of positive integers, such as the set of even positive numbers or of primes. If `A` is a subset of `S`, and if the proportion of elements of `S` below `n` that are in `A` (out of all elements of `S` below `n`) tends to 1 as `n` tends to infinity, then it can be said that almost all elements of `S` are in `A`.

Examples:

- The natural density of cofinite sets of positive integers is 1, so each of them contains almost all positive integers.
- Almost all positive integers are composite.
^{[sec 6]}^{[proof 1]} - Almost all even positive numbers can be expressed as the sum of two primes.
^{[5]}^{:489} - Almost all primes are isolated. Moreover, for every positive integer
`g`, almost all primes have prime gaps of more than`g`both to their left and to their right (no other primes between`p−g`and`p+g)`.^{[18]}

### Meaning in graph theoryEdit

In graph theory, if `A` is a set of (finite labelled) graphs, it can be said to contain almost all graphs if the proportion of graphs with `n` vertices that are in `A` tends to 1 as `n` tends to infinity.^{[19]} However, it is sometimes easier to work with probabilities,^{[20]} so the definition is reformulated as follows. The proportion of graphs with `n` vertices that are in `A` equals the probability that a random graph with `n` vertices (chosen with the uniform distribution) is in `A`, and choosing a graph in this way has the same outcome as generating a graph by flipping a coin for each pair of vertices to decide whether to connect them.^{[21]} Therefore, equivalently to the preceding definition, `A` contains almost all graphs if the probability that a coin flip-generated graph with `n` vertices is in `A` tends to 1 as `n` tends to infinity.^{[20]}^{[22]} Sometimes the latter definition is modified so that the graph is chosen randomly in some other way, where not all graphs with `n` vertices have the same probability,^{[21]} and those modified definitions are not always equivalent to the main one.

The use of the term "almost all" in graph theory is not standard; the term "asymptotically almost surely" is more commonly used for this concept.^{[20]}

Example:

- Almost all graphs are asymmetric.
^{[19]} - Almost all graphs have diameter 2.
^{[23]}

### Meaning in topologyEdit

In topology^{[24]} and especially dynamical systems theory^{[25]}^{[26]}^{[27]} (including applications in economics),^{[28]} "almost all" of a topological space's points can mean "all of the space's points but those in a meagre set". Some use a more limited definition, where a subset only contains almost all of the space's points if it contains some open dense set.^{[26]}^{[29]}^{[30]}

Example:

- Given an irreducible algebraic variety, the properties that hold for almost all points in the variety are exactly the generic properties.
^{[sec 7]}It is because in an irreducible algebraic variety equipped with the Zariski topology, all nonempty open sets are dense.

### Meaning in algebraEdit

In abstract algebra and mathematical logic, if `U` is an ultrafilter on a set `X`, "almost all elements of `X`" sometimes means "the elements of some *element* of `U`".^{[31]}^{[32]}^{[33]}^{[34]} For any partition of `X` into two disjoint sets, one of them necessarily contains almost all elements of `X`. It is possible to think of the elements of a filter on `X` as containing almost all elements of `X` even if it isn't an ultrafilter.^{[34]}

## ProofsEdit

**^**According to the prime number theorem, the number of primes less than or equal to`n`is asymptotically equal to`n`/ln(`n`). Therefore, the proportion of primes is roughly ln(`n`)/`n`, which tends to 0 as`n`tends to infinity, so the proportion of composite numbers less than or equal to`n`tends to 1 as`n`tends to infinity.^{[17]}

## ReferencesEdit

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