Projectively extended real line

Dividing by zero

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 reciprocal, 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

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

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

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, (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 R 2 .

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 R of R ^ , the following operations are defined for a ∈ R ^ , 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 a , b , c ∈ R ^ .

The following is true whenever the right-hand side is defined, for any a , b , c ∈ R ^ .

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

Intervals and topology

The concept of an interval can be extended to R ^ . 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 a , b ∈ R , a < b ):

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

R ^ and the empty set are each also an interval, as is R ^ excluding any single point.

The open intervals as base define a topology on R ^ . Sufficient for a base are the finite open intervals in R and the intervals ( b , a ) = { x ∣ x ∈ R , b < x } ∪ { ∞ } ∪ { x ∣ x ∈ R , x < a } for all a , b ∈ R such that a < b .

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 R .

Interval arithmetic

Interval arithmetic extends to R ^ from R . 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 a , b ∈ R ^ :

irrespective of whether either interval includes 0 and ∞ .

Calculus

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

Neighbourhoods

Let x ∈ R ^ , A ⊆ R ^ .

Basic definitions of limits

Let f : R ^ → R ^ , p ∈ R ^ , L ∈ R ^ .

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 x ∈ B implies f ( x ) ∈ A .

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 x ∈ B implies f ( x ) ∈ A .

It can be shown that lim x → p f ( x ) = L if and only if both lim x → p + f ( x ) = L and lim x → p − f ( x ) = L .

Comparison with limits in R {displaystyle mathbb {R} }

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

Extended definition of limits

Let A ⊆ R ^ . Then p is a limit point of A if and only if every neighbourhood of p includes a point y ∈ A such that y ≠ x .

Let f : R ^ → R ^ , A ⊆ R ^ , L ∈ R ^ , p ∈ R ^ , 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 x ∈ A ∩ C implies f ( x ) ∈ B .

This corresponds to the regular topological definition of continuity, applied to the subspace topology on A ∪ { p } , and the restriction of f to A ∪ { p } .

Continuity

Let

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

Let

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

An interesting feature is that every rational function P(x)/Q(x), where P(x) and Q(x) have no common factor, is continuous in R ^ . Also, If tan is extended so that

then tan is continuous in R . However, many elementary functions, such as trigonometric and exponential functions, are discontinuous at ∞ . For example, sin is continuous in R but discontinuous at ∞ .

Thus 1/x is continuous on R ^ but not on the affinely extended real number system R ¯ . Conversely, the function arctan can be extended continuously on R ¯ , but not on R ^ .

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(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.