Geometry 

History of geometry 
Research areas

Important concepts
Point · Line · Perpendicular · Parallel · Line segment · Ray · Plane · Length · Area · Volume · Vertex · Angle · Congruence · Similarity · Polygon · Triangle · Altitude · Hypotenuse · Pythagorean theorem · Quadrilateral · Trapezoid · Kite · Parallelogram (Rhomboid, Rectangle, Rhombus, Square) · Diagonal · Symmetry · Curve · Circle · Area of a disk · Circumference · Diameter · Cylinder · Sphere · Pyramid · Dimensions (one, two, three, four)

Geometers
Aryabhata · Ahmes · Apolonius · Archimedes · Baudhayana · Bolyai · Brahmagupta · Euclid · Pythagoras · Khayyám · Descartes · Pascal · Euler · Gauss · Ibn alYasamin · Jyeṣṭhadeva · Kātyāyana · Lobachevsky · Manava · Minggatu · Riemann · Klein · Parameshvara · Poincaré · Sijzi · Hilbert · Minkowski · Cartan · Veblen · Sakabe Kōhan · Gromov · Atiyah · Virasena · Yang Hui · Yasuaki Aida · Zhang Heng

Geometry (Greek γεωμετρία; geo = earth, metria = measure) arose as the field of knowledge dealing with spatial relationships. Geometry was one of the two fields of premodern mathematics, the other being the study of numbers (arithmetic).
Classic geometry was focused in compass and straightedge constructions. Geometry was revolutionized by Euclid, who introduced mathematical rigor and the axiomatic method still in use today. His book, The Elements is widely considered the most influential textbook of all time, and was known to all educated people in the West until the middle of the 20th century.^{[1]}
In modern times, geometric concepts have been generalized to a high level of abstraction and complexity, and have been subjected to the methods of calculus and abstract algebra, so that many modern branches of the field are barely recognizable as the descendants of early geometry. (See areas of mathematics and algebraic geometry.)
Contents
Early geometry
The earliest recorded beginnings of geometry can be traced to early peoples, who discovered obtuse triangles in the ancient Indus Valley (see Harappan Mathematics), and ancient Babylonia (see Babylonian mathematics) from around 3000 BC. Early geometry was a collection of empirically discovered principles concerning lengths, angles, areas, and volumes, which were developed to meet some practical need in surveying, construction, astronomy, and various crafts. Among these were some surprisingly sophisticated principles, and a modern mathematician might be hard put to derive some of them without the use of calculus. For example, both the Egyptians and the Babylonians were aware of versions of the Pythagorean theorem about 1500 years before Pythagoras; the Egyptians had a correct formula for the volume of a frustum of a square pyramid;
Egyptian geometry
Main article: Egyptian mathematics
The ancient Egyptians knew that they could approximate the area of a circle as follows:^{[2]}



 Area of Circle ≈ [ (Diameter) x 8/9 ]^{2}.^{[2]}


Problem 30 of the Ahmes papyrus uses these methods to calculate the area of a circle, according to a rule that the area is equal to the square of 8/9 of the circle's diameter. This assumes that π is 4×(8/9)² (or 3.160493...), with an error of slightly over 0.63 percent. This value was slightly less accurate than the calculations of the Babylonians (25/8 = 3.125, within 0.53 percent), but was not otherwise surpassed until Archimedes' approximation of 211875/67441 = 3.14163, which had an error of just over 1 in 10,000.
Interestingly, Ahmes knew of the modern 22/7 as an approximation for pi, and used it to split a hekat, hekat x 22/x x 7/22 = hekat; however, Ahmes continued to use the traditional 256/81 value for pi for computing his hekat volume found in a cylinder.
Problem 48 involved using a square with side 9 units. This square was cut into a 3x3 grid. The diagonal of the corner squares were used to make an irregular octagon with an area of 63 units. This gave a second value for π of 3.111...
The two problems together indicate a range of values for Pi between 3.11 and 3.16.
Problem 14 in the Moscow Mathematical Papyrus gives the only ancient example finding the volume of a frustum of a pyramid, describing the correct formula:
Babylonian geometry
Main article: Babylonian mathematics
The Babylonians may have known the general rules for measuring areas and volumes. They measured the circumference of a circle as three times the diameter and the area as onetwelfth the square of the circumference, which would be correct if π is estimated as 3. The volume of a cylinder was taken as the product of the base and the height, however, the volume of the frustum of a cone or a square pyramid was incorrectly taken as the product of the height and half the sum of the bases. The Pythagorean theorem was also known to the Babylonians. Also, there was a recent discovery in which a tablet used π as 3 and 1/8. The Babylonians are also known for the Babylonian mile, which was a measure of distance equal to about seven miles today. This measurement for distances eventually was converted to a timemile used for measuring the travel of the Sun, therefore, representing time.^{[3]}
Greek geometry
See also: Greek mathematics
Classical Greek geometry
For the ancient Greek mathematicians, geometry was the crown jewel of their sciences, reaching a completeness and perfection of methodology that no other branch of their knowledge had attained. They expanded the range of geometry to many new kinds of figures, curves, surfaces, and solids; they changed its methodology from trialanderror to logical deduction; they recognized that geometry studies "eternal forms", or abstractions, of which physical objects are only approximations; and they developed the idea of the "axiomatic method", still in use today.
Thales and Pythagoras
Thales (635543 BC) of Miletus (now in southwestern Turkey), was the first to whom deduction in mathematics is attributed. There are five geometric propositions for which he wrote deductive proofs, though his proofs have not survived. Pythagoras (582496 BC) of Ionia, and later, Italy, then colonized by Greeks, may have been a student of Thales, and traveled to Babylon and Egypt. The theorem that bears his name may not have been his discovery, but he was probably one of the first to give a deductive proof of it. He gathered a group of students around him to study mathematics, music, and philosophy, and together they discovered most of what high school students learn today in their geometry courses. In addition, they made the profound discovery of incommensurable lengths and irrational numbers.
Plato
Plato (427347 BC), the philosopher most esteemed by the Greeks, had inscribed above the entrance to his famous school, "Let none ignorant of geometry enter here." Though he was not a mathematician himself, his views on mathematics had great influence. Mathematicians thus accepted his belief that geometry should use no tools but compass and straightedge – never measuring instruments such as a marked ruler or a protractor, because these were a workman’s tools, not worthy of a scholar. This dictum led to a deep study of possible compass and straightedge constructions, and three classic construction problems: how to use these tools to trisect an angle, to construct a cube twice the volume of a given cube, and to construct a square equal in area to a given circle. The proofs of the impossibility of these constructions, finally achieved in the 19th century, led to important principles regarding the deep structure of the real number system. Aristotle (384322 BC), Plato’s greatest pupil, wrote a treatise on methods of reasoning used in deductive proofs (see Logic) which was not substantially improved upon until the 19th century.
Hellenistic geometry
Euclid
Euclid (c. 325265 BC), of Alexandria, probably a student of one of Plato’s students, wrote a treatise in 13 books (chapters), titled The Elements of Geometry, in which he presented geometry in an ideal axiomatic form, which came to be known as Euclidean geometry. The treatise is not a compendium of all that the Hellenistic mathematicians knew at the time about geometry; Euclid himself wrote eight more advanced books on geometry. We know from other references that Euclid’s was not the first elementary geometry textbook, but it was so much superior that the others fell into disuse and were lost. He was brought to the university at Alexandria by Ptolemy I, King of Egypt.
The Elements began with definitions of terms, fundamental geometric principles (called axioms or postulates), and general quantitative principles (called common notions) from which all the rest of geometry could be logically deduced. Following are his five axioms, somewhat paraphrased to make the English easier to read.
 Any two points can be joined by a straight line.
 Any finite straight line can be extended in a straight line.
 A circle can be drawn with any center and any radius.
 All right angles are equal to each other.
 If two straight lines in a plane are crossed by another straight line (called the transversal), and the interior angles between the two lines and the transversal lying on one side of the transversal add up to less than two right angles, then on that side of the transversal, the two lines extended will intersect (also called the parallel postulate).
Archimedes
Archimedes (287212 BC), of Syracuse, Sicily, when it was a Greek citystate, is often considered to be the greatest of the Greek mathematicians, and occasionally even named as one of the three greatest of all time (along with Isaac Newton and Carl Friedrich Gauss). Had he not been a mathematician, he would still be remembered as a great physicist, engineer, and inventor. In his mathematics, he developed methods very similar to the coordinate systems of analytic geometry, and the limiting process of integral calculus. The only element lacking for the creation of these fields was an efficient algebraic notation in which to express his concepts^{[citation needed]}.
After Archimedes
After Archimedes, Hellenistic mathematics began to decline. There were a few minor stars yet to come, but the golden age of geometry was over. Proclus (410485), author of Commentary on the First Book of Euclid, was one of the last important players in Hellenistic geometry. He was a competent geometer, but more importantly, he was a superb commentator on the works that preceded him. Much of that work did not survive to modern times, and is known to us only through his commentary. The Roman Republic and Empire that succeeded and absorbed the Greek citystates produced excellent engineers, but no mathematicians of note.
The great Library of Alexandria was later burned. There is a growing consensus among historians that the Library of Alexandria likely suffered from several destructive events, but that the destruction of Alexandria's pagan temples in the late 4th century was probably the most severe and final one. The evidence for that destruction is the most definitive and secure. Caesar's invasion may well have led to the loss of some 40,00070,000 scrolls in a warehouse adjacent to the port (as Luciano Canfora argues, they were likely copies produced by the Library intended for export), but it is unlikely to have affected the Library or Museum, given that there is ample evidence that both existed later.
Civil wars, decreasing investments in maintenance and acquisition of new scrolls and generally declining interest in nonreligious pursuits likely contributed to a reduction in the body of material available in the Library, especially in the 4th century. The Serapeum was certainly destroyed by Theophilus in 391, and the Museum and Library may have fallen victim to the same campaign.
Indian geometry
See also: Indian mathematics
Vedic period
The Satapatha Brahmana (ninth century BC) contains rules for ritual geometric constructions that are similar to the Sulba Sutras.^{[4]}
The Śulba Sūtras (literally, "Aphorisms of the Chords" in Vedic Sanskrit) (c. 700400 BC) list rules for the construction of sacrificial fire altars.^{[5]} Most mathematical problems considered in the Śulba Sūtras spring from "a single theological requirement,"^{[6]} that of constructing fire altars which have different shapes but occupy the same area. The altars were required to be constructed of five layers of burnt brick, with the further condition that each layer consist of 200 bricks and that no two adjacent layers have congruent arrangements of bricks.^{[6]}
According to (Hayashi 2005, p. 363), the Śulba Sūtras contain "the earliest extant verbal expression of the Pythagorean Theorem in the world, although it had already been known to the Old Babylonians."
The diagonal rope (akṣṇayārajju) of an oblong (rectangle) produces both which the flank (pārśvamāni) and the horizontal (tiryaṇmānī) <ropes> produce separately."^{[7]}
Since the statement is a sūtra, it is necessarily compressed and what the ropes produce is not elaborated on, but the context clearly implies the square areas constructed on their lengths, and would have been explained so by the teacher to the student.^{[7]}
They contain lists of Pythagorean triples,^{[8]} which are particular cases of Diophantine equations.^{[9]} They also contain statements (that with hindsight we know to be approximate) about squaring the circle and "circling the square."^{[10]}
Baudhayana (c. eighth century BC) composed the Baudhayana Sulba Sutra, the bestknown Sulba Sutra, which contains examples of simple Pythagorean triples, such as: , , , , and ^{[11]} as well as a statement of the Pythagorean theorem for the sides of a square: "The rope which is stretched across the diagonal of a square produces an area double the size of the original square."^{[11]} It also contains the general statement of the Pythagorean theorem (for the sides of a rectangle): "The rope stretched along the length of the diagonal of a rectangle makes an area which the vertical and horizontal sides make together."^{[11]}
According to mathematician S. G. Dani, the Babylonian cuneiform tablet Plimpton 322 written c. 1850 BC^{[12]} "contains fifteen Pythagorean triples with quite large entries, including (13500, 12709, 18541) which is a primitive triple,^{[13]} indicating, in particular, that there was sophisticated understanding on the topic" in Mesopotamia in 1850 BC. "Since these tablets predate the Sulbasutras period by several centuries, taking into account the contextual appearance of some of the triples, it is reasonable to expect that similar understanding would have been there in India."^{[14]} Dani goes on to say:
"As the main objective of the Sulvasutras was to describe the constructions of altars and the geometric principles involved in them, the subject of Pythagorean triples, even if it had been well understood may still not have featured in the Sulvasutras. The occurrence of the triples in the Sulvasutras is comparable to mathematics that one may encounter in an introductory book on architecture or another similar applied area, and would not correspond directly to the overall knowledge on the topic at that time. Since, unfortunately, no other contemporaneous sources have been found it may never be possible to settle this issue satisfactorily."^{[14]}
In all, three Sulba Sutras were composed. The remaining two, the Manava Sulba Sutra composed by Manava (fl. 750650 BC) and the Apastamba Sulba Sutra, composed by Apastamba (c. 600 BC), contained results similar to the Baudhayana Sulba Sutra.
Classical period
In the Bakhshali manuscript, there is a handful of geometric problems (including problems about volumes of irregular solids). The Bakhshali manuscript also "employs a decimal place value system with a dot for zero."^{[15]} Aryabhata's Aryabhatiya (499) includes the computation of areas and volumes.
Brahmagupta wrote his astronomical work Brāhma Sphuṭa Siddhānta in 628. Chapter 12, containing 66 Sanskrit verses, was divided into two sections: "basic operations" (including cube roots, fractions, ratio and proportion, and barter) and "practical mathematics" (including mixture, mathematical series, plane figures, stacking bricks, sawing of timber, and piling of grain).^{[16]} In the latter section, he stated his famous theorem on the diagonals of a cyclic quadrilateral:^{[16]}
Brahmagupta's theorem: If a cyclic quadrilateral has diagonals that are perpendicular to each other, then the perpendicular line drawn from the point of intersection of the diagonals to any side of the quadrilateral always bisects the opposite side.
Chapter 12 also included a formula for the area of a cyclic quadrilateral (a generalization of Heron's formula), as well as a complete description of rational triangles (i.e. triangles with rational sides and rational areas).
Brahmagupta's formula: The area, A, of a cyclic quadrilateral with sides of lengths a, b, c, d, respectively, is given by
where s, the semiperimeter, given by:
Brahmagupta's Theorem on rational triangles: A triangle with rational sides and rational area is of the form:
for some rational numbers and .^{[17]}
Chinese geometry
See also: Chinese mathematics
The first definitive work (or at least oldest existent) on geometry in China was the Mo Jing, the Mohist canon of the early philosopher Mozi (470390 BC). It was compiled years after his death by his later followers around the year 330 BC.^{[18]} Although the Mo Jing is the oldest existent book on geometry in China, there is the possibility that even older written material exists. However, due to the infamous Burning of the Books in the political maneauver by the Qin Dynasty ruler Qin Shihuang (r. 221210 BC), multitudes of written literature created before his time was purged. In addition, the Mo Jing presents geometrical concepts in mathematics that are perhaps too advanced not to have had a previous geometrical base or mathematic background to work upon.
The Mo Jing described various aspects of many fields associated with physical science, and provided a small wealth of information on mathematics as well. It provided an 'atomic' definition of the geometric point, stating that a line is separated into parts, and the part which has no remaining parts (i.e. cannot be divided into smaller parts) and thus forms the extreme end of a line is a point.^{[18]} Much like Euclid's first and third definitions and Plato's 'beginning of a line', the Mo Jing stated that "a point may stand at the end (of a line) or at its beginning like a headpresentation in childbirth. (As to its invisibility) there is nothing similar to it."^{[19]} Similar to the atomists of Democritus, the Mo Jing stated that a point is the smallest unit, and cannot be cut in half, since 'nothing' cannot be halved.^{[19]} It stated that two lines of equal length will always finish at the same place,^{[19]} while providing definitions for the comparison of lengths and for parallels,^{[20]} along with principles of space and bounded space.^{[21]} It also described the fact that planes without the quality of thickness cannot be piled up since they cannot mutually touch.^{[22]} The book provided definitions for circumference, diameter, and radius, along with the definition of volume.^{[23]}
The Han Dynasty (202 BC220 AD) period of China witnessed a new flourishing of mathematics. One of the oldest Chinese mathematical texts to present geometric progressions was the Suàn shù shū of 186 BC, during the Western Han era. The mathematician, inventor, and astronomer Zhang Heng (78139 AD) used geometrical formulas to solve mathematical problems. Although rough estimates for pi (π) were given in the Zhou Li (compiled in the 2nd century BC),^{[24]} it was Zhang Heng who was the first to make a concerted effort at creating a more accurate formula for pi. Zhang Heng approximated pi as 730/232 (or approx 3.1466), although he used another formula of pi in finding a spherical volume, using the square root of 10 (or approx 3.162) instead. Zu Chongzhi (429500 AD) improved the accuracy of the approximation of pi to between 3.1415926 and 3.1415927, with ^{355}⁄_{113} (密率, Milü, detailed approximation) and ^{22}⁄_{7} (约率, Yuelü, rough approximation) being the other notable approximation.^{[25]} In comparison to later works, the formula for pi given by the French mathematician Franciscus Vieta (15401603) fell halfway between Zu's approximations.
The Nine Chapters on the Mathematical Art
The Nine Chapters on the Mathematical Art, the title of which first appeared by 179 AD on a bronze inscription, was edited and commented on by the 3rd century mathematician Liu Hui from the Kingdom of Cao Wei. This book included many problems where geometry was applied, such as finding surface areas for squares and circles, the volumes of solids in various threedimensional shapes, and included the use of the Pythagorean theorem. The book provided illustrated proof for the Pythagorean theorem,^{[26]} contained a written dialogue between of the earlier Duke of Zhou and Shang Gao on the properties of the right angle triangle and the Pythagorean theorem, while also referring to the astronomical gnomon, the circle and square, as well as measurements of heights and distances.^{[27]} The editor Liu Hui listed pi as 3.141014 by using a 192 sided polygon, and then calculated pi as 3.14159 using a 3072 sided polygon. This was more accurate than Liu Hui's contemporary Wang Fan, a mathematician and astronomer from Eastern Wu, would render pi as 3.1555 by using ^{142}⁄_{45}.^{[28]} Liu Hui also wrote of mathematical surveying to calculate distance measurements of depth, height, width, and surface area. In terms of solid geometry, he figured out that a wedge with rectangular base and both sides sloping could be broken down into a pyramid and a tetrahedral wedge.^{[29]} He also figured out that a wedge with trapezoid base and both sides sloping could be made to give two tetrahedral wedges separated by a pyramid.^{[29]} Furthermore, Liu Hui described Cavalieri's principle on volume, as well as Gaussian elimination. From the Nine Chapters, it listed the following geometrical formulas that were known by the time of the Former Han Dynasty (202 BCE–9 CE).
Areas for the^{[30]}

Volumes for the^{[29]}



Continuing the geometrical legacy of ancient China, there were many later figures to come, including the famed astronomer and mathematician Shen Kuo (10311095 CE), Yang Hui (12381298) who discovered Pascal's Triangle, Xu Guangqi (15621633), and many others.
Islamic geometry
See also: Islamic mathematics
Although the Muslim mathematicians are most famed for their work on algebra, number theory and number systems, they also made considerable contributions to geometry, trigonometry and mathematical astronomy, and were responsible for the development of algebraic geometry. Geometrical magnitudes were treated as "algebraic objects" by most Muslim mathematicians however.
AlMahani (born 820) conceived the idea of reducing geometrical problems such as duplicating the cube to problems in algebra. AlKaraji (born 953) completely freed algebra from geometrical operations and replaced them with the arithmetical type of operations which are at the core of algebra today.
Thabit family and other early geometers
Thābit ibn Qurra (known as Thebit in Latin) (born 836) contributed to a number of areas in mathematics, where he played an important role in preparing the way for such important mathematical discoveries as the extension of the concept of number to (positive) real numbers, integral calculus, theorems in spherical trigonometry, analytic geometry, and nonEuclidean geometry. In astronomy Thabit was one of the first reformers of the Ptolemaic system, and in mechanics he was a founder of statics. An important geometrical aspect of Thabit's work was his book on the composition of ratios. In this book, Thabit deals with arithmetical operations applied to ratios of geometrical quantities. The Greeks had dealt with geometric quantities but had not thought of them in the same way as numbers to which the usual rules of arithmetic could be applied. By introducing arithmetical operations on quantities previously regarded as geometric and nonnumerical, Thabit started a trend which led eventually to the generalisation of the number concept.
In some respects, Thabit is critical of the ideas of Plato and Aristotle, particularly regarding motion. It would seem that here his ideas are based on an acceptance of using arguments concerning motion in his geometrical arguments. Another important contribution Thabit made to geometry was his generalization of the Pythagorean theorem, which he extended from special right triangles to all triangles in general, along with a general proof.^{[31]}
Ibrahim ibn Sinan ibn Thabit (born 908), who introduced a method of integration more general than that of Archimedes, and alQuhi (born 940) were leading figures in a revival and continuation of Greek higher geometry in the Islamic world. These mathematicians, and in particular Ibn alHaytham, studied optics and investigated the optical properties of mirrors made from conic sections.
Astronomy, timekeeping and geography provided other motivations for geometrical and trigonometrical research. For example Ibrahim ibn Sinan and his grandfather Thabit ibn Qurra both studied curves required in the construction of sundials. Abu'lWafa and Abu Nasr Mansur both applied spherical geometry to astronomy.
Geometric architecture
Recent discoveries have shown that geometrical quasicrystal patterns were first employed in the girih tiles found in medieval Islamic architecture dating back over five centuries ago. In 2007, Professor Peter Lu of Harvard University and Professor Paul Steinhardt of Princeton University published a paper in the journal Science suggesting that girih tilings possessed properties consistent with selfsimilar fractal quasicrystalline tilings such as the Penrose tilings, predating them by five centuries.^{[32]}^{[33]}
Modern geometry
The 17th century
When Europe began to emerge from its Dark Ages, the Hellenistic and Islamic texts on geometry found in Islamic libraries were translated from Arabic into Latin. The rigorous deductive methods of geometry found in Euclid’s Elements of Geometry were relearned, and further development of geometry in the styles of both Euclid (Euclidean geometry) and Khayyam (algebraic geometry) continued, resulting in an abundance of new theorems and concepts, many of them very profound and elegant.
In the early 17th century, there were two important developments in geometry. The first and most important was the creation of analytic geometry, or geometry with coordinates and equations, by René Descartes (1596–1650) and Pierre de Fermat (1601–1665). This was a necessary precursor to the development of calculus and a precise quantitative science of physics. The second geometric development of this period was the systematic study of projective geometry by Girard Desargues (1591–1661). Projective geometry is the study of geometry without measurement, just the study of how points align with each other. There had been some early work in this area by Hellenistic geometers, notably Pappus (c. 340). The greatest flowering of the field occurred with JeanVictor Poncelet (1788–1867).
In the late 17th century, calculus was developed independently and almost simultaneously by Isaac Newton (1642–1727) and Gottfried Wilhelm Leibniz (1646–1716). This was the beginning of a new field of mathematics now called analysis. Though not itself a branch of geometry, it is applicable to geometry, and it solved two families of problems that had long been almost intractable: finding tangent lines to odd curves, and finding areas enclosed by those curves. The methods of calculus reduced these problems mostly to straightforward matters of computation.
The 18th and 19th centuries
NonEuclidean geometry
The old problem of proving Euclid’s Fifth Postulate, the "Parallel Postulate", from his first four postulates had never been forgotten. Beginning not long after Euclid, many attempted demonstrations were given, but all were later found to be faulty, through allowing into the reasoning some principle which itself had not been proved from the first four postulates. Though Omar Khayyám was also unsuccessful in proving the parallel postulate, his criticisms of Euclid's theories of parallels and his proof of properties of figures in nonEuclidean geometries contributed to the eventual development of nonEuclidean geometry. By 1700 a great deal had been discovered about what can be proved from the first four, and what the pitfalls were in attempting to prove the fifth. Saccheri, Lambert, and Legendre each did excellent work on the problem in the 18th century, but still fell short of success. In the early 19th century, Gauss, Johann Bolyai, and Lobatchewsky, each independently, took a different approach. Beginning to suspect that it was impossible to prove the Parallel Postulate, they set out to develop a selfconsistent geometry in which that postulate was false. In this they were successful, thus creating the first nonEuclidean geometry. By 1854, Bernhard Riemann, a student of Gauss, had applied methods of calculus in a groundbreaking study of the intrinsic (selfcontained) geometry of all smooth surfaces, and thereby found a different nonEuclidean geometry. This work of Riemann later became fundamental for Einstein's theory of relativity.
It remained to be proved mathematically that the nonEuclidean geometry was just as selfconsistent as Euclidean geometry, and this was first accomplished by Beltrami in 1868. With this, nonEuclidean geometry was established on an equal mathematical footing with Euclidean geometry.
While it was now known that different geometric theories were mathematically possible, the question remained, "Which one of these theories is correct for our physical space?" The mathematical work revealed that this question must be answered by physical experimentation, not mathematical reasoning, and uncovered the reason why the experimentation must involve immense (interstellar, not earthbound) distances. With the development of relativity theory in physics, this question became vastly more complicated.
Introduction of mathematical rigor
All the work related to the Parallel Postulate revealed that it was quite difficult for a geometer to separate his logical reasoning from his intuitive understanding of physical space, and, moreover, revealed the critical importance of doing so. Careful examination had uncovered some logical inadequacies in Euclid's reasoning, and some unstated geometric principles to which Euclid sometimes appealed. This critique paralleled the crisis occurring in calculus and analysis regarding the meaning of infinite processes such as convergence and continuity. In geometry, there was a clear need for a new set of axioms, which would be complete, and which in no way relied on pictures we draw or on our intuition of space. Such axioms were given by David Hilbert in 1894 in his dissertation Grundlagen der Geometrie (Foundations of Geometry). Some other complete sets of axioms had been given a few years earlier, but did not match Hilbert's in economy, elegance, and similarity to Euclid's axioms.
Analysis situs, or topology
In the mid18th century, it became apparent that certain progressions of mathematical reasoning recurred when similar ideas were studied on the number line, in two dimensions, and in three dimensions. Thus the general concept of a metric space was created so that the reasoning could be done in more generality, and then applied to special cases. This method of studying calculus and analysisrelated concepts came to be known as analysis situs, and later as topology. The important topics in this field were properties of more general figures, such as connectedness and boundaries, rather than properties like straightness, and precise equality of length and angle measurements, which had been the focus of Euclidean and nonEuclidean geometry. Topology soon became a separate field of major importance, rather than a subfield of geometry or analysis.
The 20th century
Developments in algebraic geometry included the study of curves and surfaces over finite fields as demonstrated by the works of among others André Weil, Alexander Grothendieck, and JeanPierre Serre as well as over the real or complex numbers. Finite geometry itself, the study of spaces with only finitely many points, found applications in coding theory and cryptography. With the advent of the computer, new disciplines such as computational geometry or digital geometry deal with geometric algorithms, discrete representations of geometric data, and so forth.
Timeline
Main article: Timeline of geometry
See also
Wikisource has original text related to this article: 
 Flatland, a book by "A. Square" about two– and threedimensional space, to understand the concept of four dimensions
 History of mathematics
 Important publications in geometry.
 Interactive geometry software
 List of geometry topics
Notes
 ^ Howard Eves, An Introduction to the History of Mathematics, Saunders, 1990, ISBN 0030295580 p. 141: "No work, except The Bible, has been more widely used...."
 ^ ^{a} ^{b} Ray C. Jurgensen, Alfred J. Donnelly, and Mary P. Dolciani. Editorial Advisors Andrew M. Gleason, Albert E. Meder, Jr. Modern School Mathematics: Geometry (Student's Edition). Houghton Mifflin Company, Boston, 1972, p. 52. ISBN 0395131022. Teachers Edition ISBN 0395131030.
 ^ Eves, Chapter 2.
 ^ A. Seidenberg, 1978. The origin of mathematics. Archive for the history of Exact Sciences, vol 18.
 ^ (Staal 1999)
 ^ ^{a} ^{b} (Hayashi 2003, p. 118)
 ^ ^{a} ^{b} (Hayashi 2005, p. 363)
 ^ Pythagorean triples are triples of integers with the property: . Thus, , , etc.
 ^ (Cooke 2005, p. 198): "The arithmetic content of the Śulva Sūtras consists of rules for finding Pythagorean triples such as (3, 4, 5), (5, 12, 13), (8, 15, 17), and (12, 35, 37). It is not certain what practical use these arithmetic rules had. The best conjecture is that they were part of religious ritual. A Hindu home was required to have three fires burning at three different altars. The three altars were to be of different shapes, but all three were to have the same area. These conditions led to certain "Diophantine" problems, a particular case of which is the generation of Pythagorean triples, so as to make one square integer equal to the sum of two others."
 ^ (Cooke 2005, pp. 199–200): "The requirement of three altars of equal areas but different shapes would explain the interest in transformation of areas. Among other transformation of area problems the Hindus considered in particular the problem of squaring the circle. The Bodhayana Sutra states the converse problem of constructing a circle equal to a given square. The following approximate construction is given as the solution.... this result is only approximate. The authors, however, made no distinction between the two results. In terms that we can appreciate, this construction gives a value for π of 18 (3 − 2√2), which is about 3.088."
 ^ ^{a} ^{b} ^{c} (Joseph 2000, p. 229)
 ^ Mathematics Department, University of British Columbia, The Babylonian tabled Plimpton 322.
 ^ Three positive integers form a primitive Pythagorean triple if and if the highest common factor of is 1. In the particular Plimpton322 example, this means that and that the three numbers do not have any common factors. However some scholars have disputed the Pythagorean interpretation of this tablet; see Plimpton 322 for details.
 ^ ^{a} ^{b} (Dani 2003)
 ^ (Hayashi 2005, p. 371)
 ^ ^{a} ^{b} (Hayashi 2003, pp. 121–122)
 ^ (Stillwell 2004, p. 77)
 ^ ^{a} ^{b} Needham, Volume 3, 91.
 ^ ^{a} ^{b} ^{c} Needham, Volume 3, 92.
 ^ Needham, Volume 3, 9293.
 ^ Needham, Volume 3, 93.
 ^ Needham, Volume 3, 9394.
 ^ Needham, Volume 3, 94.
 ^ Needham, Volume 3, 99.
 ^ Needham, Volume 3, 101.
 ^ Needham, Volume 3, 22.
 ^ Needham, Volume 3, 21.
 ^ Needham, Volume 3, 100.
 ^ ^{a} ^{b} ^{c} Needham, Volume 3, 98–99.
 ^ Needham, Volume 3, 98.
 ^ Sayili, Aydin (1960). "Thabit ibn Qurra's Generalization of the Pythagorean Theorem". Isis 51 (1): 35–37. doi:10.1086/348837.
 ^ Peter J. Lu and Paul J. Steinhardt (2007), "Decagonal and Quasicrystalline Tilings in Medieval Islamic Architecture", Science 315 (5815): 1106–1110, Bibcode:2007Sci...315.1106L, doi:10.1126/science.1135491, PMID 17322056.
 ^ Supplemental figures
References
 Cooke, Roger (2005), The History of Mathematics:, New York: WileyInterscience, 632 pages, ASIN 0471444596, ISBN 0471444596
 Dani, S. G. (July 25, 2003), "Pythogorean Triples in the Sulvasutras", Current Science 85 (2): 219–224
 Eder, Michelle (2000), Views of Euclid's Parallel Postulate in Ancient Greece and in Medieval Islam, Rutgers University, retrieved 20080123
 Hayashi, Takao (2003), "Indian Mathematics", in GrattanGuinness, Ivor, Companion Encyclopedia of the History and Philosophy of the Mathematical Sciences 1, Baltimore, MD: The Johns Hopkins University Press, 976 pages, pp. 118–130, ISBN 0801873967
 Hayashi, Takao (2005), "Indian Mathematics", in Flood, Gavin, The Blackwell Companion to Hinduism, Oxford: Basil Blackwell, 616 pages, pp. 360–375, ISBN 9781405132510
 Joseph, G. G. (2000), The Crest of the Peacock: The NonEuropean Roots of Mathematics, Princeton, NJ: Princeton University Press, 416 pages, ISBN 0691006598
 Katz, Victor J. (1998), History of Mathematics: An Introduction, AddisonWesley, ISBN 0321016181, OCLC 38199387 60154481
 Needham, Joseph (1986), Science and Civilization in China: Volume 3, Mathematics and the Sciences of the Heavens and the Earth, Taipei: Caves Books Ltd
 Rozenfeld, Boris A. (1988), A History of NonEuclidean Geometry: Evolution of the Concept of a Geometric Space, Springer Science+Business Media, ISBN 0387964584, OCLC 15550634 230166667 230980046 77693662
 Smith, John D. (1992), "The Remarkable Ibn alHaytham", The Mathematical Gazette (Mathematical Association) 76 (475): 189–198, doi:10.2307/3620392, JSTOR 3620392
 Staal, Frits (1999), "Greek and Vedic Geometry", Journal of Indian Philosophy, 27 (1–2): 105–127, doi:10.1023/A:1004364417713
 Stillwell, John (2004), Berlin and New York: Mathematics and its History (2 ed.), Springer, 568 pages, ISBN 0387953361
External links
 Islamic Geometry
 Geometry in the 19th Century at the Stanford Encyclopedia of Philosophy
 Arabic mathematics : forgotten brilliance?