# Projectively extended real line

This article does not cite any sources. (December 2014) (Learn how and when to remove this template message) |

In real analysis, the **projectively extended real line** (also called the one-point compactification of the real line), is the extension of the real line, , by a point denoted ∞. It is thus the set with the standard arithmetic operations extended where possible, and is sometimes denoted by The added point is called the point at infinity, because it is considered as a neighbour of both ends of the real line. More precisely, the point at infinity is the limit of every sequence of real numbers whose absolute values are increasing and unbounded.

The projectively extended real line may be identified with the projective line over the reals in which three points have been assigned specific values (e.g. 0, 1 and ∞). The projectively extended real line must not be confused with the extended real number line, in which +∞ and −∞ are distinct.

## Dividing by zero[edit]

Unlike most mathematical models of the intuitive concept of 'number', this structure allows division by zero:

for nonzero *a*. In particular 1/0 = ∞, and moreover 1/∞ = 0, making the reciprocal function, *x* ↦ 1/*x*, a total function in this structure. The structure, however, is not a field, and none of the binary arithmetic operations are total, as witnessed for example by 0 ⋅ ∞ being undefined despite the reciprocal being total. It has usable interpretations, however – for example, in geometry, a vertical line has *infinite* slope.

## Extensions of the real line[edit]

The projectively extended real line extends the field of real numbers in the same way that the Riemann sphere extends the field of complex numbers, by adding a single point called conventionally .

In contrast, the extended real number line (also called the two-point compactification of the real line) distinguishes between and .

## Order[edit]

The order relation cannot be extended to in a meaningful way. Given a number , there is no convincing argument to define either or that . Since can't be compared with any of the other elements, there's no point in retaining this relation on . However, order on is used in definitions in .

## Geometry[edit]

Fundamental to the idea that ∞ is a point *no different from any other* is the way the real projective line is a homogeneous space, in fact homeomorphic to a circle. For example the general linear group of 2×2 real invertible matrices has a transitive action on it. The group action may be expressed by Möbius transformations, (also called linear fractional transformations), with the understanding that when the denominator of the linear fractional transformation is 0, the image is ∞.

The detailed analysis of the action shows that for any three distinct points *P*, *Q* and *R*, there is a linear fractional transformation taking *P* to 0, *Q* to 1, and *R* to ∞ that is, the group of linear fractional transformations is triply transitive on the real projective line. This cannot be extended to 4-tuples of points, because the cross-ratio is invariant.

The terminology projective line is appropriate, because the points are in 1-to-1 correspondence with one-dimensional linear subspaces of .

## Arithmetic operations[edit]

### Motivation for arithmetic operations[edit]

The arithmetic operations on this space are an extension of the same operations on reals. A motivation for the new definitions is the limits of functions of real numbers.

### Arithmetic operations that are defined[edit]

In addition to the standard operations on the subset of , the following operations are defined for , with exceptions as indicated:

### Arithmetic operations that are left undefined[edit]

The following expressions cannot be motivated by considering limits of real functions, and no definition of them allows the statement of the standard algebraic properties to be retained unchanged in form for all defined cases.^{[a]} Consequently, they are left undefined:

## Algebraic properties[edit]

The following equalities mean: *Either both sides are undefined, or both sides are defined and equal.* This is true for any .

The following is true whenever the right-hand side is defined, for any .

In general, all laws of arithmetic that are valid for are also valid for whenever all the occurring expressions are defined.

## Intervals and topology[edit]

The concept of an interval can be extended to . However, since it is an unordered set, the interval has a slightly different meaning. The definitions for closed intervals are as follows (it is assumed that ):

With the exception of when the end-points are equal, the corresponding open and half-open intervals are defined by removing the respective endpoints.

and the empty set are each also an interval, as is excluding any single point.^{[b]}

The open intervals as base define a topology on . Sufficient for a base are the finite open intervals in and the intervals for all such that .

As said, the topology is homeomorphic to a circle. Thus it is metrizable corresponding (for a given homeomorphism) to the ordinary metric on this circle (either measured straight or along the circle). There is no metric which is an extension of the ordinary metric on .

## Interval arithmetic[edit]

Interval arithmetic extends to from . The result of an arithmetic operation on intervals is always an interval, except when the intervals with a binary operation contain incompatible values leading to an undefined result.^{[c]} In particular, we have, for every :

irrespective of whether either interval includes and .

## Calculus[edit]

The tools of calculus can be used to analyze functions of . The definitions are motivated by the topology of this space.

### Neighbourhoods[edit]

Let .

*A*is a neighbourhood of*x*, if and only if*A*contains an open interval*B*and .*A*is a right-sided neighbourhood of x, if and only if there is such that*A*contains .*A*is a left-sided neighbourhood of x, if and only if there is such that*A*contains .*A*is a (right-sided, left-sided) punctured neighbourhood of*x*, if and only if there is such that*B*is a (right-sided, left-sided) neighbourhood of x, and .

### Limits[edit]

#### Basic definitions of limits[edit]

Let .

The limit of *f(x)* as *x* approaches *p* is *L*, denoted

if and only if for every neighbourhood *A* of *L*, there is a punctured neighbourhood *B* of *p*, such that implies .

The one-sided limit of *f(x)* as *x* approaches *p* from the right (left) is *L*, denoted

if and only if for every neighbourhood *A* of *L*, there is a right-sided (left-sided) punctured neighbourhood *B* of *p*, such that implies .

It can be shown that if and only if both and .

#### Comparison with limits in [edit]

The definitions given above can be compared with the usual definitions of limits of real functions. In the following statements, , the first limit is as defined above, and the second limit is in the usual sense:

- is equivalent to .
- is equivalent to .
- is equivalent to .
- is equivalent to .
- is equivalent to .
- is equivalent to .

#### Extended definition of limits[edit]

Let . Then *p* is a limit point of *A* if and only if every neighbourhood of *p* includes a point such that .

Let , *p* a limit point of *A*. The limit of *f(x)* as *x* approaches *p* through *A* is *L*, if and only if for every neighbourhood *B* of *L*, there is a punctured neighbourhood *C* of *p*, such that implies .

This corresponds to the regular topological definition of continuity, applied to the subspace topology on , and the restriction of *f* to .

### Continuity[edit]

The function

is continuous at *p* if and only if *f* is defined at *p* and

If the function

is continuous in *A* if and only if, for every , *f* is defined at *p* and the limit of *f*(*x*) as *x* tends to *p* through *A* is *f*(*p*).

Every rational function *P*(*x*)/*Q*(*x*), where *P* and *Q* are polynomials, can be prolongated, in a unique way, to a function from to that is continuous in . In particular, this is the case of polynomial functions, which take the value at if they are not constant.

Also, if the tangent function tan is extended so that

then tan is continuous in but cannot be prolongated further to a function that is continuous in

Many elementary functions that are continuous in cannot be prolongated to functions that are continuous in This is the case, for example, of the exponential function and all trigonometric functions. For example, the sine function is continuous in but it cannot be made continuous at As seen above, the tangent function can be prolongated to a function that is continuous in but this function cannot be made continuous at

Many discontinuous functions that become continuous when the codomain is extended to remain discontinuous if the codomain is extended to the affinely extended real number system This is the case of the function On the other hand, some functions that are continuous in and discontinuous at become continuous if the domain is extended to This is the case of the arc tangent.

## As a projective range[edit]

When the real projective line is considered in the context of the real projective plane, then the consequences of Desargues' theorem are implicit. In particular, the construction of the projective harmonic conjugate relation between points is part of the structure of the real projective line. For instance, given any pair of points, the point at infinity is the projective harmonic conjugate of their midpoint.

As projectivities preserve the harmonic relation, they form the automorphisms of the real projective line. The projectivities are described algebraically as homographies, since the real numbers form a ring, according to the general construction of a projective line over a ring. Collectively they form the group PGL(2,R).

The projectivities which are their own inverses are called involutions. A **hyperbolic involution** has two fixed points. Two of these correspond to elementary, arithmetic operations on the real projective line: negation and reciprocation. Indeed, 0 and ∞ are fixed under negation, while 1 and −1 are fixed under reciprocation.

## Notes[edit]

**^**An extension does however exist in which all the algebraic properties, when restricted to defined operations in , resolve to the standard rules: see Wheel theory.**^**If consistency of complementation is required, such that and for all (where the interval on either side is defined), all intervals excluding and may be naturally represented using this notation, with being interpreted as , and half-open intervals with equal endpoints, e.g. , remaining undefined.**^**For example, the ratio of intervals contains in both intervals, and since is undefined, the result of division of these intervals is undefined.