Projectively extended real line


In real analysis, the projectively extended real line, is the extension of the set of the real numbers, 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. The projectively extended real line must not be confused with the extended real number line, in which and are distinct.

Dividing by zero

Unlike most mathematical models of the intuitive concept of 'number', this structure allows division by zero:
for nonzero a. In particular, and moreover, making reciprocal,, 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 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

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 distinguishes between and.

Order

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

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,, 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

Motivation for arithmetic operations

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

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

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. Consequently, they are left undefined:

Algebraic properties

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

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 :
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.
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 to the ordinary metric on this circle. There is no metric which is an extension of the ordinary metric on.

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. In particular, we have, for every :
irrespective of whether either interval includes and.

Calculus

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

Neighbourhoods

Let.

Basic definitions of limits

Let.
The limit of f 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 as x approaches p from the right is L, denoted
if and only if for every neighbourhood A of L, there is a right-sided punctured neighbourhood B of p, such that implies.
It can be shown that if and only if both and.

Comparison with limits in \mathbb{R}

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:
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 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

The function
is continuous at if and only if is defined at and
If the function
is continuous in if and only if, for every, is defined at and the limit of as tends to through is.
Every rational function, where and 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 is extended so that
then 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

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.
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.