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Inversive geometry

Inversive geometry is a branch of plane geometry that studies transformations and properties invariant under inversion in a circle, extending the Euclidean plane by including a point at infinity and treating straight lines as circles of infinite radius known as generalized circles. The core transformation, circle inversion, maps a point P (distinct from the circle's center O) to a point P' lying on the ray from O through P such that the product of the distances OP and OP' equals the square of the circle's radius r^2. This inversion is an involution, meaning it is its own inverse and applying it twice yields the identity transformation, and it exchanges points inside and outside the circle while fixing points on the circle itself. Key properties include the preservation of angles between curves and the mapping of generalized circles to generalized circles, with lines not passing through O inverting to circles passing through O, and circles passing through O inverting to lines not through O. Inversion also preserves orthogonality, mapping pairs of orthogonal circles to orthogonal pairs, and maintains cross-ratios and separation properties of points. Inversive geometry finds applications in solving classical problems involving circle configurations, such as , which describes closed chains of tangent circles between two given non-intersecting circles, and in visualizing non-Euclidean geometries by transforming hyperbolic or spherical models into the Euclidean plane. It also aids in , such as generating tilings and patterns via inversive transformations, and has connections to through representations in the . The systematic development of inversive geometry emerged in the 19th century, with Swiss mathematician Jakob Steiner (1796–1863) among the first to extensively employ inversion techniques for geometric problem-solving, building on earlier ideas like those of Julius Plücker. Later works, such as H.S.M. Coxeter and S.L. Greitzer's Geometry Revisited (1967), further popularized its methods in education and olympiad problems.

Core Concepts

Definition of Circle Inversion

Circle inversion is a geometric transformation defined in the Euclidean plane with respect to a given circle, serving as the foundational operation in inversive geometry. Consider a circle centered at a point O with radius k > 0. For any point P in the plane distinct from O, the inverse point P' is the unique point lying on the ray starting at O and passing through P such that the product of the distances OP \cdot OP' = k^2. This relation ensures that points inside the circle map outside and vice versa, with points on the circle remaining fixed. The transformation establishes a bijection between the Euclidean plane excluding the center O (often called the pole of inversion) and itself, as the mapping is invertible: applying the inversion twice returns the original point. This one-to-one correspondence preserves the structure of the plane minus the pole, providing a powerful tool for studying geometric configurations by relating distant points through the circle's reference. The of inversion was introduced by the in his 1826 paper "Einige geometrische Betrachtungen," where it was employed as a for synthetic geometric constructions, particularly involving the power of a point and circle similitudes.

Geometric Construction of Inversion

The geometric construction of the inverse point P' of a point P with respect to a circle centered at O with radius r can be performed using a compass and straightedge, relying on the power of a point and properties of right triangles. This method exploits the fact that the inverse point lies on the ray from O through P such that OP \cdot OP' = r^2, and it is particularly straightforward for points P outside the circle. For P outside the circle, the construction proceeds as follows:
  1. Draw the line segment \overline{OP}.
  2. Construct the perpendicular bisector of \overline{OP} to locate the midpoint M.
  3. With center M and radius \overline{OM}, draw a circle; this circle has diameter \overline{OP} and intersects the original circle at two points T and T' (symmetric across \overline{OP}).
  4. Select one intersection point, say T.
  5. Construct the perpendicular from T to the line \overline{OP}; the foot of this perpendicular is the inverse point P'.
This yields right triangles formed by the radius to T and the tangent from P to T, ensuring the geometric mean relation holds. For P inside the circle, a modified procedure is required to avoid direct tangents:
  1. Draw the ray \overrightarrow{OP}.
  2. Locate point Q on \overrightarrow{OP} such that \overline{OP} \cong \overline{PQ}.
  3. Construct the perpendicular bisector of \overline{OQ} to find its intersection T with the original circle.
  4. Draw the ray \overrightarrow{OT}.
  5. Locate point U on \overrightarrow{OP} such that \overline{OT} \cong \overline{TU}.
  6. Construct the perpendicular bisector of \overline{OU}; its intersection with \overrightarrow{OP} is P'.
These steps create auxiliary right triangles that enforce the inversion relation through congruent segments and bisectors. An alternative method, known as Dutta's construction, provides an efficient approach using intersecting chords and harmonic properties, applicable regardless of whether P is inside or outside the circle. To construct the inverse A' of point A with respect to a circle \Omega_1 centered at O_1 with radius O_1P (where P is chosen such that O_1P > O_1A and A, O_1, P are non-collinear):
  1. Draw the ray from O_1 through A, intersecting \Omega_1 at B.
  2. Construct a circle centered at B with radius \overline{BA}.
  3. Construct a circle centered at P with radius \overline{PA}; let C be one intersection point of these two circles (other than A).
  4. Draw the line \overline{PC}, which intersects the ray from O_1 through A at A'.
This method leverages the inversion's preservation of angles and the harmonic division induced by the intersecting circles, simplifying the process in configurations where tangents are awkward. Both constructions yield the same unique inverse point P', as demonstrated by the similarity of right triangles in the standard method (where \triangle OTP \sim \triangle OT{P'}, with ratio r / OP = OP' / r, confirming OP \cdot OP' = r^2) and the chord power equality in Dutta's approach (where the intersections preserve the cross-ratio harmonic property aligned with the inversion definition).

Fundamental Properties of Inversion

Inversion with respect to a circle of radius k centered at the origin in the plane is defined such that for a point P = (x, y) at distance r = \sqrt{x^2 + y^2} from the center, its inverse P' = (x', y') lies on the same ray from the origin and satisfies OP \cdot OP' = k^2. This leads to the coordinate equations x' = \frac{k^2 x}{x^2 + y^2} and y' = \frac{k^2 y}{x^2 + y^2}, derived by scaling the position vector of P by the factor \frac{k^2}{r^2}. These equations highlight the non-linear nature of inversion, as distances from the center are scaled inversely proportional to the square of the original distance, contrasting sharply with linear transformations like similarities or isometries that preserve ratios uniformly. A key property of inversion is its conformality: it preserves angles between intersecting curves, mapping them to curves that intersect at the same angle, though it reverses the orientation of figures. This local preservation of shape makes inversion useful for studying geometric configurations, as infinitesimal angles remain unchanged in magnitude. The transformation integrates naturally with the power of a point theorem, where the power of P with respect to the inverting circle—defined as OP^2 - k^2—relates directly to the inversion process, since the product OP \cdot OP' = k^2 implies that the power of the inverse point P' is the negative of the power of P scaled by \frac{k^2}{OP^2}. Regarding fixed points, inversion maps the center of the circle to infinity and vice versa, leaving no finite fixed point there, but every point on the inverting circle itself is fixed individually, as their distance from the center equals k, satisfying OP \cdot OP' = k^2 with P' = P. Thus, the circle is invariant as a set under the transformation.

Transformations in Inversive Geometry

Inversion as a Mapping

Inversion serves as a fundamental transformation in inversive geometry, mapping points in the Euclidean plane relative to a fixed circle, typically centered at the origin for simplicity. Consider a circle of radius k centered at the origin O. For a point P with position vector \vec{P} and distance r = \|\vec{P}\| from O, the inverse point P' lies on the ray from O through P such that OP \cdot OP' = k^2. This geometric condition arises from the power of a point theorem or similar triangles in the construction, where the inversion preserves the product of distances along radial lines. Algebraically, in Cartesian coordinates, the mapping transforms (x, y) to (x', y') via x' = \frac{k^2 x}{x^2 + y^2}, \quad y' = \frac{k^2 y}{x^2 + y^2}. This formula derives directly from the vector form \vec{P'} = \frac{k^2}{\|\vec{P}\|^2} \vec{P}, which scales the position vector inversely proportional to its squared length, demonstrating radial scaling that inverts distances from the center. The transformation is undefined at the origin, where the denominator vanishes. The mapping extends the plane by incorporating behavior at infinity: points approaching the center O are sent to infinity, while points at large distances map near O, effectively interchanging finite and infinite regions. This property compactifies the plane into the inversive plane by adjoining a single point at infinity, resolving singularities and unifying lines and circles as "generalized circles" that may pass through this ideal point. Inversion is self-inverse, meaning applying the transformation twice yields the identity map (except at the center, which remains undefined). Thus, the inverse mapping is the inversion itself, as substituting the formula into itself recovers the original coordinates for any P \neq O. Unlike reflections, which are isometries preserving distances globally, inversion is not an isometry; it distorts lengths by a position-dependent factor. However, it acts as a local similarity, scaling distances uniformly in all directions at each point while preserving angles, due to its conformal nature.

Compositions and Group Structure

The inversive group, often denoted as the group of inversive transformations, is generated by the set of all circle inversions together with the Euclidean isometries of the plane, which include translations, rotations, and reflections. This generation mirrors the way reflections generate the Euclidean group, but extends it to include non-isometric mappings that preserve angles up to sign. The resulting group acts on the extended complex plane \hat{\mathbb{C}} = \mathbb{C} \cup \{\infty\}, treating lines as circles through infinity. A key closure property is that the composition of two circle inversions yields a Möbius transformation, which is an orientation-preserving mapping of the form z \mapsto \frac{az + b}{cz + d} with ad - bc \neq 0. Conversely, every Möbius transformation can be expressed as a composition of at most four inversions, highlighting the foundational role of inversions in generating the group. In the complex plane, a single circle inversion with center at the origin and radius k corresponds to the anti-Möbius map z \mapsto \frac{k^2}{\bar{z}}, where \bar{z} denotes the complex conjugate; more generally, inversions in arbitrary circles take this anti-conformal form after conjugation by a Möbius transformation. The full inversive group thus consists of all Möbius transformations and their anti-Möbius counterparts (involving conjugation), forming a structure isomorphic to the orthogonal group O(3,1) acting on the Riemann sphere. Inversions are inherently orientation-reversing, as they reverse the direction of angles, leading the full inversive group to include both conformal (orientation-preserving) and anti-conformal (orientation-reversing) maps. This dual nature arises because composing an even number of inversions preserves orientation (yielding Möbius transformations), while an odd number reverses it. The group acts transitively on the set of all circles (including lines as circles through infinity), meaning any circle can be mapped to any other via an element of the group, which underscores its utility in studying circle packings and orthogonal configurations.

Anticonformal Nature

In inversive geometry, circle inversion exhibits an anticonformal nature, preserving the magnitude of angles between intersecting curves while reversing their orientation, or handedness. This means that if two curves intersect at an angle θ, their images under inversion intersect at the same angle θ, but the clockwise or counterclockwise sense is flipped. Such transformations are sense-reversing conformal maps, where the Jacobian determinant is negative at every point. The local behavior of inversion involves a similarity factor that scales lengths by |k² / r²|, where k is the radius of the inversion circle and r is the distance from the inversion center to the point of interest. This factor indicates that infinitesimally small shapes are mapped to similar shapes, up to the orientation reversal and the position-dependent scaling. For areas, the scaling is |(k² / r²)²|, reflecting the conformal preservation of shape up to reflection. The orientation reversal is evident in the Jacobian determinant derived from the coordinate transformation. For an inversion in the unit circle centered at the origin, the mapping is given by (x, y) ↦ (x / (x² + y²), y / (x² + y²)). The Jacobian matrix has partial derivatives leading to a determinant of -1 / (x² + y²)² = -1 / r⁴, where the negative sign confirms the reversal. In general, for radius k, the determinant is -(k⁴ / r⁴), obtained by scaling the unit case. This negative value distinguishes inversion from orientation-preserving transformations. In contrast to direct conformal maps like rotations, translations, and dilations—which have positive Jacobian determinants and preserve orientation—inversions are inherently orientation-reversing. While direct maps form the special conformal group, inversions generate anti-conformal elements, and compositions involving an odd number of inversions remain orientation-reversing. The anticonformal property of inversions proves valuable in geometric constructions and proofs, enabling the establishment of equivalence between figures up to reflection. For example, in the classical Apollonius problem of constructing a circle tangent to three given circles, inversion can reduce the configuration to a simpler case involving reflection across a line, facilitating the solution via straightedge and compass methods.

Geometric Objects and Examples

Behavior of Circles and Lines

In inversive geometry, circle inversion preserves the family of generalized circles, which encompass both ordinary circles and straight lines (the latter treated as circles of infinite radius). The key theorem states that any inversion maps a generalized circle to another generalized circle. This property arises because the inversion transformation, defined with respect to a circle of radius r centered at point O, sends points P to their inverses P' such that OP \cdot OP' = r^2, and the image of a set of points lying on a generalized circle satisfies the equation of another such curve. A proof outline relies on the power of a point theorem and geometric properties like the inscribed angle theorem (angle in a semicircle is $90^\circ). Consider a generalized circle \gamma and the inversion center O. The power of O with respect to \gamma determines the mapping: if \gamma intersects the inverting circle orthogonally, the power is r^2, preserving the curve. For lines through O, the ray structure ensures the image coincides with the original line, as each point and its inverse lie on the same ray. Lines not through O map to circles passing through O, demonstrated by constructing similar triangles or using the fact that the inverse points form a curve where angles subtended by diameters are right angles, yielding a circular arc. Similarly, circles not through O invert to circles not through O, via a homothety centered at O with ratio related to the power. Special cases highlight the theorem's utility. Circles passing through the inversion center O map to straight lines not containing O, as the inverse points lie on a line parallel to the tangent at O or determined by limiting behavior. Conversely, straight lines not through O map to circles through O. Circles orthogonal to the inverting circle are invariant as sets under inversion, meaning their images coincide with themselves, though points are permuted along the curve; this follows from the orthogonality condition ensuring the power equals r^2, preserving intersection angles of $90^\circ. Lines through O are also invariant. The pole and polar relation provides another perspective on these transformations with respect to the inverting circle \Gamma(O, r). For any point P \neq O, the polar of P is the line perpendicular to the ray OP passing through the inverse point P', satisfying OP \cdot OP' = r^2. This line is the locus of points harmonic to P with respect to intersection points on secants from P. Inversion interchanges poles and polars in the sense that applying the transformation maps a point to a location on its own polar and transforms polar lines into curves whose poles relate symmetrically, embodying the duality inherent in the geometry. If P lies on the polar of a point Q, then Q lies on the polar of P, and inversion preserves such incidences.

Applications in Two Dimensions

Inversive geometry provides powerful tools for solving classical problems in the Euclidean plane, particularly those involving circles and tangency conditions. One prominent application is the solution to the Apollonius problem of constructing circles tangent to three given circles. By performing an inversion with respect to a suitable circle, the problem can be transformed into a simpler configuration, such as finding lines tangent to three transformed circles or concentric circles tangent to adjusted ones, reducing the quadratic equations to linear constructions. For instance, in the case of three mutually tangent circles, inversion simplifies the search for the inner or outer Soddy circles by mapping the tangency conditions to parallel tangents or concentric setups. This method yields up to eight solutions, depending on the combinations of internal and external tangencies. Inversion also simplifies the study of orthogonal circles and radical axes. Circles that intersect orthogonally—meaning at right angles—are mapped to other orthogonal circles under inversion, preserving their intersection angles and facilitating proofs of tangency or power properties. The radical axis of two circles, defined as the locus of points with equal power relative to both, transforms under inversion into the radical axis of the images, often straightening complex intersections into lines perpendicular to the line of centers. This is particularly useful for analyzing families of circles, where inversion can convert intersecting orthogonal pairs into concentric circles, easing computations of common tangents or mid-circles. Concrete examples illustrate these transformations vividly. Inverting a limaçon with respect to a circle centered at its node produces a cardioid, revealing shared properties with conic sections such as envelopes or pedal curves, derived from the inversion of an ellipse or hyperbola. Similarly, a system of coaxial circles—those sharing a common radical axis and intersecting at two fixed points—can be inverted, with the center at one intersection point, to yield a pencil of parallel lines, simplifying the analysis of their envelope or orthogonal trajectories. The construction of inverse curves exemplifies these principles, especially for lines. Consider a straight line not passing through the inversion center O. Its inverse is a circle passing through O, with the original line serving as the radical axis of this circle and the inversion circle. To construct it explicitly, select two points P and Q on the line; their inverses P' and Q' lie on the resulting circle, which can be drawn by finding the circle through O, P', and Q'. This circle's diameter is perpendicular to the original line at the foot of the perpendicular from O, highlighting how inversion interchanges lines and circles while preserving angles and incidence.

Applications in Three Dimensions

In three-dimensional space, sphere inversion is defined analogously to circle inversion in the plane, mapping a point \mathbf{P} to its inverse \mathbf{P}' with respect to a sphere of center \mathbf{O} and radius k, such that \mathbf{P} and \mathbf{P}' lie on the same ray from \mathbf{O} and OP \cdot OP' = k^2. For a sphere centered at the origin with radius r, the transformation of a point \mathbf{x} is given by \mathbf{x}' = \frac{r^2}{\|\mathbf{x}\|^2} \mathbf{x}. This mapping is conformal, preserving angles up to orientation reversal, and extends the properties of planar inversive geometry to volumes. Under sphere inversion, spheres and planes map to spheres or planes, treating planes as spheres of infinite radius. Specifically, a sphere not passing through the inversion center maps to another sphere, while one passing through the center maps to a plane; conversely, planes not through the center map to spheres, and those through the center map to themselves. This property simplifies the study of configurations involving multiple spheres, as inversion preserves incidence and tangency relations. Common geometric objects transform into more complex surfaces under sphere inversion. A right circular cylinder inverts to a Dupin cyclide, a quartic surface characterized by lines of curvature that are circles. Similarly, a right circular cone inverts to a Dupin cyclide, often a double horn cyclide where circles intersect at the vertex and origin. A torus inverts to a Dupin cyclide, such as a ring cyclide or a self-intersecting spindle cyclide depending on the inversion sphere's position relative to the torus. These transformations highlight how inversion generates families of algebraic surfaces from simpler quadrics, with Dupin cyclides notable for their applications in modeling canal surfaces and conformal parametrizations. Sphere inversion finds practical use in simplifying three-dimensional intersection and tangency problems. For instance, constructing spheres tangent to two given spheres and a plane—where the given elements are mutually tangent—becomes tractable by inverting with respect to a sphere centered at a common tangency point. This inversion maps the given spheres to parallel planes and the plane to a sphere, reducing the problem to finding tangent spheres in the inverted space, whose images yield solutions in the original configuration while preserving tangency. Such techniques extend classical problems like the Apollonius problem to 3D, aiding in geometric constructions and dynamic geometry software implementations.

Extensions and Generalizations

Inversion in Higher Dimensions

In n-dimensional Euclidean space \mathbb{R}^n, inversion is generalized as a transformation with respect to an (n-1)-dimensional hypersphere of radius k centered at the origin. For a point x \in \mathbb{R}^n \setminus \{0\}, the inverse point x' is given by the formula x' = \frac{k^2}{\|x\|^2} x, where \|x\|^2 = \sum_{i=1}^n x_i^2 is the squared Euclidean norm. This mapping sends points inside the hypersphere to the exterior and vice versa, while fixing points on the hypersphere itself. For a hypersphere centered at an arbitrary point c, the transformation is composed with a translation: first shift by -c, apply the origin-centered inversion, then shift back by +c. A key property of this inversion is its preservation of hyperspheres and hyperplanes: it maps (n-1)-dimensional hyperspheres not passing through the center to other hyperspheres, and those passing through the center to hyperplanes (and vice versa). This follows from the fact that the equation of a hypersphere in \mathbb{R}^n, \|x - a\|^2 = r^2, transforms under inversion into a similar quadratic form, yielding another hypersphere or a hyperplane if the original passes through the inversion center. Such mappings maintain the conformal structure, preserving angles up to orientation reversal, which extends the foundational behaviors observed in lower dimensions like circles and lines in the plane. In four dimensions (n=4), inversions find natural expression through quaternionic structures, where points in \mathbb{R}^4 are identified with quaternions q, and inversion with respect to the unit hypersphere corresponds to the quaternion inverse q^{-1} = \bar{q} / |q|^2, facilitating computations in quaternionic analysis and hyperbolic geometries. This representation aids in studying Möbius transformations in higher dimensions. Historically, such 4D inversions have appeared in contexts bridging Euclidean geometry and early formulations of relativity, such as conformal mappings in Euclideanized spacetime models, though modern applications emphasize computational and algebraic tools over physical interpretations. Beyond three dimensions, implementing inversions computationally faces increasing challenges due to the exponential growth in coordinate complexity; for instance, explicit matrix representations or numerical stability for hypersphere equations scale poorly with n, limiting practical simulations in fields like computer graphics or optimization without specialized algebraic structures like Clifford algebras.

Axiomatic Foundations

Inversive geometry can be axiomatized as an abstract structure independent of Euclidean coordinates, with points and circles serving as primitive notions alongside an incidence relation between them. The foundational work in this direction was undertaken by Mario Pieri, who in 1911 presented a system of axioms emphasizing harmonic properties and deriving inversions as transformations that preserve circles and reverse angles. Pieri's approach, detailed in his paper "Nuovi principii di geometria delle inversioni," posits inversion as a primitive operation that maps points to points while maintaining the incidence of circles, thereby establishing inversive geometry as a hypothetical-deductive system akin to projective geometry. Building on the synthetic tradition initiated by Karl von Staudt's 1847 treatise Geometrie der Lage, which axiomatized projective geometry through incidence and collinearity without metric assumptions, inversive geometry adapts these ideas by replacing lines with circles. Von Staudt's contributions provided the methodological framework for metric-free geometries, influencing later axiomatizations that treat inversions as the core collineation-like mappings in a circular context. Modern formulations, such as those by H. L. Dorwart and others, refine this by specifying axioms for points, circles, and their intersections. A standard set of basic axioms for inversive plane geometry includes the following:
  • Incidence Axiom I1: For any three distinct points, there exists a unique circle incident with all three.
  • Intersection Axiom I2: Given distinct points P and Q, and a circle C through P but not Q, there exists a unique circle C' through Q intersecting C only at P.
  • Non-degeneracy Axiom I3: There exist four points no three of which are incident with a common circle.
These axioms ensure a rich structure where inversion, defined with respect to a circle, preserves incidence and maps circles to circles while reversing orientation, thus capturing the anticonformal nature essential to the geometry. The generalization to the inversive plane incorporates a point at infinity, treating straight lines as circles passing through this ideal point, thereby unifying lines and circles under a single primitive. In this framework, the axioms extend to include the existence of the infinite point and properties ensuring that every pair of points lies on a unique "generalized circle" (either a proper circle or a line through infinity). This construction parallels the projective plane, where the line at infinity compactifies the Euclidean plane, but in the inversive case, it emphasizes circular orthogonality and angle preservation up to sign. Compared to projective geometry, inversive geometry emerges as a circular variant, where the role of projective collineations is played by compositions of inversions and reflections, forming the Möbius group of transformations. While projective axioms focus on line incidences and the fundamental theorem of projective geometry (uniqueness of lines through two points), inversive axioms prioritize circle incidences and the theorem of Miquel, which guarantees closure under certain circle chains. This distinction highlights inversive geometry's focus on conformal properties, filling a gap in formal systems by providing a synthetic basis for circle-preserving mappings without reliance on analytic coordinates.

Invariants under Inversion

In inversive geometry, one key invariant under circle inversion is the magnitude of angles formed by the intersection of generalized circles (lines or circles). Specifically, the angle between two curves at their intersection point remains unchanged in measure after inversion, though the orientation is reversed, meaning clockwise angles become counterclockwise and vice versa. This property holds because inversion is an anticonformal mapping, preserving local angle measures while flipping the direction of traversal along the curves. For unoriented angles, which disregard direction and focus solely on magnitude, inversion fully preserves them, making it a powerful tool for studying angle-related configurations without regard to handedness. In contrast, oriented angles, which include directional information, are negated under inversion, reflecting the transformation's orientation-reversing nature similar to reflections. This distinction is crucial in applications where directional consistency matters, such as in oriented curve families. Another fundamental invariant is the cross-ratio of four points lying on a common generalized circle. The cross-ratio (A, B; C, D) for points A, B, C, D on a line or circle, defined as \frac{(A-C)/(B-C)}{(A-D)/(B-D)} (or its complex analog in the plane), remains unchanged under inversion. This invariance extends to the broader group of Möbius transformations generated by compositions of inversions, allowing the cross-ratio to serve as a complete invariant classifying quadruples of points up to inversive equivalence. For instance, four points on a circle have a real-valued cross-ratio, and inversion maps them to another set on a generalized circle with the same value, preserving harmonic properties like cross-ratios of -1. Orthogonality between circles is also preserved under inversion. If two circles intersect at right angles before inversion, their images under the transformation will intersect at right angles as well. Notably, any circle orthogonal to the reference circle of inversion is mapped to itself as a set, with points along the circle redistributed but the curve unchanged; the intersection points with the reference circle remain fixed. This self-invariance of orthogonal circles underscores their role in defining fixed structures within inversive mappings. Certain loci, such as specific pencils of circles, remain invariant as sets under inversion. A pencil of circles— a one-parameter family sharing a common radical axis or passing through two fixed points—is mapped to another pencil of the same type, but in cases like the pencil consisting of all circles orthogonal to the inversion circle, the entire family is invariant, with each member mapped to itself. Coaxial pencils, defined by circles with a common radical axis, preserve their coaxial structure post-inversion, maintaining the invariant nature of their radical axis and overall configuration. These invariant pencils provide essential frameworks for classifying circle families in inversive geometry.

Connections to Broader Geometries

Relation to the Erlangen Program

In Felix Klein's Erlangen program, introduced in 1872, geometries are classified according to the groups of transformations that preserve their fundamental structures, with each geometry defined by the invariants under its associated group. Inversive geometry emerges as the geometry of circle-preserving transformations in the plane, often termed the "geometry of reciprocal radii," where the primary elements are points, circles, and spheres, treating lines and planes as degenerate circles passing through a point at infinity. This framework positions inversive geometry as a conformal geometry, emphasizing angle preservation alongside the mapping of generalized circles (circles or lines) to themselves. The transformation group of inversive geometry is generated by two key subgroups: similarities, which include dilations, rotations, translations, and reflections that preserve shapes and sizes up to scaling, and reciprocations, corresponding to inversions with respect to circles. Compositions of these generators yield the full group of circle-preserving maps, equivalent to the Möbius transformations in the complex plane, which act transitively on the set of circles and points. This group structure ensures that inversive geometry studies properties invariant under these transformations, such as tangency and the power of a point with respect to a circle. In higher dimensions, inversive geometry extends naturally, with its group acting on spheres in three-dimensional space, bridging to projective geometry by identifying the plane with a quadric surface via stereographic projection, where inversions correspond to projective transformations. This connection highlights inversive geometry's role as an intermediary between Euclidean similarities and the broader conformal geometries, including those explored in Lie's sphere geometry, where oriented spheres are treated as fundamental elements preserved by the group.

Links to Hyperbolic Geometry

Inversive geometry provides a foundational framework for modeling hyperbolic geometry through the Poincaré disk model, where the unit disk in the complex plane serves as the hyperbolic plane, and the boundary circle acts as the absolute conic at infinity. In this model, inversions with respect to circles orthogonal to the boundary circle generate the reflections that form the building blocks of hyperbolic isometries. Specifically, such an inversion maps the interior of the unit disk to itself and acts as a reflection across the corresponding hyperbolic line, which is the arc of the inverting circle inside the disk. The group of hyperbolic isometries is then generated by compositions of these inversions, preserving the hyperbolic structure while maintaining conformality. The hyperbolic metric in the Poincaré disk arises naturally from these inversive transformations, scaling the Euclidean metric to account for the curvature. The infinitesimal distance element is given by ds = \frac{|dz|}{1 - |z|^2}, where z is a point in the unit disk |z| < 1, and this form ensures that distances grow logarithmically toward the boundary, reflecting the infinite extent of hyperbolic space. This metric can be derived by considering the effect of inversions on lengths near the boundary, where Euclidean distances are distorted to produce constant negative curvature of -1. Geodesics in this model are the arcs of circles (or diameters) orthogonal to the boundary circle, as these are precisely the curves invariant under the relevant inversions and represent the shortest paths in the hyperbolic sense. One key advantage of this inversive approach to the Poincaré disk is its conformal preservation of angles, allowing Euclidean circle drawings to directly visualize hyperbolic phenomena without distortion in local shapes, though areas and lengths are scaled. This representation bridges classical inversive techniques with non-Euclidean geometry, offering explicit derivations of models that were historically developed more abstractly, and it highlights how the absolute boundary enforces the hyperbolic axioms through orthogonality conditions. Coxeter emphasized this connection, showing how the inversive plane embeds the hyperbolic plane as a natural extension.

Stereographic Projection and Conformal Mappings

Stereographic projection provides a conformal mapping between the unit sphere in three-dimensional space, excluding the north pole, and the equatorial plane. Formally, for a point (x, y, z) on the unit sphere S^2 with north pole at (0, 0, 1), the projection onto the plane z = 0 is given by the complex coordinate \zeta = \frac{x + i y}{1 - z}, where the plane is identified with the complex plane \mathbb{C}. This mapping, dating back to ancient Greek astronomers but formalized in modern geometry, establishes a bijective correspondence that excludes only the projection point, which corresponds to the point at infinity in the plane. In the context of inversive geometry, stereographic projection can be expressed as a composition of an inversion in the plane and a reflection across the equatorial plane of the sphere. Specifically, the projection aligns with circle inversion properties by interchanging points symmetric with respect to the equator, such that if two points on the sphere are reflections in the equatorial plane, their projections are inversive images under a circle inversion in the plane with power equal to the square of the radius. This equivalence highlights how stereographic projection inherits the circle-preserving and angle-preserving qualities of inversions. The conformal nature of stereographic projection ensures it preserves angles globally, meaning that the angle between two curves on the sphere equals the angle between their images on the plane; this follows from the mapping being a local similarity transformation, where the derivative at each point is a scalar multiple of an orthogonal transformation. Consequently, it maps circles and lines on the sphere (great circles) to circles and lines in the plane, maintaining orthogonality where applicable. This projection extends naturally to higher dimensions, providing a homeomorphism from the n-sphere S^n minus a point to \mathbb{R}^n, with analogous formulas derived from radial lines through the pole. In specific applications, such as parametrizing points on S^5, stereographic projection facilitates the use of six-sphere-like coordinates by embedding the 5-dimensional manifold into a 6-dimensional framework for computational or kinematic purposes. A key application in complex analysis is the compactification of the complex plane to the Riemann sphere, where stereographic projection identifies \mathbb{C} with S^2 minus the north pole, adding the point at infinity to form a compact Riemann surface suitable for uniform treatment of meromorphic functions.