Irrational number
In mathematics, the irrational numbers (in- + rational) are all the real numbers that are not rational numbers. That is, irrational numbers cannot be expressed as the ratio of two integers. When the ratio of lengths of two line segments is an irrational number, the line segments are also described as being incommensurable, meaning that they share no "measure" in common, that is, there is no length ("the measure"), no matter how short, that could be used to express the lengths of both of the two given segments as integer multiples of itself.
Among irrational numbers are the ratio
Like all real numbers, irrational numbers can be expressed in positional notation, notably as a decimal number. In the case of irrational numbers, the decimal expansion does not terminate, nor end with a repeating sequence. For example, the decimal representation of π starts with 3.14159, but no finite number of digits can represent π exactly, nor does it repeat. Conversely, a decimal expansion that terminates or repeats must be a rational number. These are provable properties of rational numbers and positional number systems and are not used as definitions in mathematics.
Irrational numbers can also be expressed as non-terminating continued fractions (which in some cases are periodic), and in many other ways.
As a consequence of
History
Ancient Greece
The first proof of the existence of irrational numbers is usually attributed to a Pythagorean (possibly Hippasus of Metapontum),[4] who probably discovered them while identifying sides of the pentagram.[5] The Pythagorean method would have claimed that there must be some sufficiently small, indivisible unit that could fit evenly into one of these lengths as well as the other. Hippasus in the 5th century BC, however, was able to deduce that there was no common unit of measure, and that the assertion of such an existence was a contradiction. He did this by demonstrating that if the
- Start with an isosceles right triangle with side lengths of integers a, b, and c. The ratio of the hypotenuse to a leg is represented by c:b.
- Assume a, b, and c are in the smallest possible terms (i.e. they have no common factors).
- By the Pythagorean theorem: c2 = a2+b2 = b2+b2 = 2b2. (Since the triangle is isosceles, a = b).
- Since c2 = 2b2, c2 is divisible by 2, and therefore even.
- Since c2 is even, c must be even.
- Since c is even, dividing c by 2 yields an integer. Let y be this integer (c = 2y).
- Squaring both sides of c = 2y yields c2 = (2y)2, or c2 = 4y2.
- Substituting 4y2 for c2 in the first equation (c2 = 2b2) gives us 4y2= 2b2.
- Dividing by 2 yields 2y2 = b2.
- Since y is an integer, and 2y2 = b2, b2 is divisible by 2, and therefore even.
- Since b2 is even, b must be even.
- We have just shown that both b and c must be even. Hence they have a common factor of 2. However, this contradicts the assumption that they have no common factors. This contradiction proves that c and b cannot both be integers and thus the existence of a number that cannot be expressed as a ratio of two integers.[6]
Greek mathematicians termed this ratio of incommensurable magnitudes alogos, or inexpressible. Hippasus, however, was not lauded for his efforts: according to one legend, he made his discovery while out at sea, and was subsequently thrown overboard by his fellow Pythagoreans 'for having produced an element in the universe which denied the... doctrine that all phenomena in the universe can be reduced to whole numbers and their ratios.'[7] Another legend states that Hippasus was merely exiled for this revelation. Whatever the consequence to Hippasus himself, his discovery posed a very serious problem to Pythagorean mathematics, since it shattered the assumption that numbers and geometry were inseparable; a foundation of their theory.
The discovery of incommensurable ratios was indicative of another problem facing the Greeks: the relation of the discrete to the continuous. This was brought to light by Zeno of Elea, who questioned the conception that quantities are discrete and composed of a finite number of units of a given size. Past Greek conceptions dictated that they necessarily must be, for "whole numbers represent discrete objects, and a commensurable ratio represents a relation between two collections of discrete objects",[8] but Zeno found that in fact "[quantities] in general are not discrete collections of units; this is why ratios of incommensurable [quantities] appear... .[Q]uantities are, in other words, continuous".[8] What this means is that contrary to the popular conception of the time, there cannot be an indivisible, smallest unit of measure for any quantity. In fact, these divisions of quantity must necessarily be infinite. For example, consider a line segment: this segment can be split in half, that half split in half, the half of the half in half, and so on. This process can continue infinitely, for there is always another half to be split. The more times the segment is halved, the closer the unit of measure comes to zero, but it never reaches exactly zero. This is just what Zeno sought to prove. He sought to prove this by formulating four paradoxes, which demonstrated the contradictions inherent in the mathematical thought of the time. While Zeno's paradoxes accurately demonstrated the deficiencies of contemporary mathematical conceptions, they were not regarded as proof of the alternative. In the minds of the Greeks, disproving the validity of one view did not necessarily prove the validity of another, and therefore, further investigation had to occur.
The next step was taken by Eudoxus of Cnidus, who formalized a new theory of proportion that took into account commensurable as well as incommensurable quantities. Central to his idea was the distinction between magnitude and number. A magnitude "...was not a number but stood for entities such as line segments, angles, areas, volumes, and time which could vary, as we would say, continuously. Magnitudes were opposed to numbers, which jumped from one value to another, as from 4 to 5".[9] Numbers are composed of some smallest, indivisible unit, whereas magnitudes are infinitely reducible. Because no quantitative values were assigned to magnitudes, Eudoxus was then able to account for both commensurable and incommensurable ratios by defining a ratio in terms of its magnitude, and proportion as an equality between two ratios. By taking quantitative values (numbers) out of the equation, he avoided the trap of having to express an irrational number as a number. "Eudoxus' theory enabled the Greek mathematicians to make tremendous progress in geometry by supplying the necessary logical foundation for incommensurable ratios".[10] This incommensurability is dealt with in Euclid's Elements, Book X, Proposition 9. It was not until Eudoxus developed a theory of proportion that took into account irrational as well as rational ratios that a strong mathematical foundation of irrational numbers was created.[11]
As a result of the distinction between number and magnitude, geometry became the only method that could take into account incommensurable ratios. Because previous numerical foundations were still incompatible with the concept of incommensurability, Greek focus shifted away from numerical conceptions such as algebra and focused almost exclusively on geometry. In fact, in many cases, algebraic conceptions were reformulated into geometric terms. This may account for why we still conceive of x2 and x3 as x squared and x cubed instead of x to the second power and x to the third power. Also crucial to Zeno's work with incommensurable magnitudes was the fundamental focus on deductive reasoning that resulted from the foundational shattering of earlier Greek mathematics. The realization that some basic conception within the existing theory was at odds with reality necessitated a complete and thorough investigation of the axioms and assumptions that underlie that theory. Out of this necessity, Eudoxus developed his method of exhaustion, a kind of reductio ad absurdum that "...established the deductive organization on the basis of explicit axioms..." as well as "...reinforced the earlier decision to rely on deductive reasoning for proof".[12] This method of exhaustion is the first step in the creation of calculus.
Theodorus of Cyrene proved the irrationality of the surds of whole numbers up to 17, but stopped there probably because the algebra he used could not be applied to the square root of 17.[13]
India
Geometrical and mathematical problems involving irrational numbers such as square roots were addressed very early during the Vedic period in India. There are references to such calculations in the Samhitas, Brahmanas, and the Shulba Sutras (800 BC or earlier).[14]
It is suggested that the concept of irrationality was implicitly accepted by Indian mathematicians since the 7th century BC, when Manava (c. 750 – 690 BC) believed that the square roots of numbers such as 2 and 61 could not be exactly determined.[15] Historian Carl Benjamin Boyer, however, writes that "such claims are not well substantiated and unlikely to be true".[16]
Later, in their treatises, Indian mathematicians wrote on the arithmetic of surds including addition, subtraction, multiplication, rationalization, as well as separation and extraction of square roots.[17]
Mathematicians like Brahmagupta (in 628 AD) and Bhāskara I (in 629 AD) made contributions in this area as did other mathematicians who followed. In the 12th century Bhāskara II evaluated some of these formulas and critiqued them, identifying their limitations.
During the 14th to 16th centuries,
Islamic World
In the
"It will be a rational (magnitude) when we, for instance, say 10, 12, 3%, 6%, etc., because its value is pronounced and expressed quantitatively. What is not rational is irrational and it is impossible to pronounce and represent its value quantitatively. For example: the roots of numbers such as 10, 15, 20 which are not squares, the sides of numbers which are not cubes etc."
In contrast to Euclid's concept of magnitudes as lines, Al-Mahani considered integers and fractions as rational magnitudes, and square roots and cube roots as irrational magnitudes. He also introduced an arithmetical approach to the concept of irrationality, as he attributes the following to irrational magnitudes:[20]
"their sums or differences, or results of their addition to a rational magnitude, or results of subtracting a magnitude of this kind from an irrational one, or of a rational magnitude from it."
The
Many of these concepts were eventually accepted by European mathematicians sometime after the
Modern period
The 17th century saw
Continued fractions, closely related to irrational numbers (and due to Cataldi, 1613), received attention at the hands of Euler, and at the opening of the 19th century were brought into prominence through the writings of Joseph-Louis Lagrange. Dirichlet also added to the general theory, as have numerous contributors to the applications of the subject.
Examples
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Square roots
The
General roots
The proof above[ is irrational.
Logarithms
Perhaps the numbers most easy to prove irrational are certain logarithms. Here is a proof by contradiction that log2 3 is irrational (log2 3 ≈ 1.58 > 0).
Assume log2 3 is rational. For some positive integers m and n, we have
It follows that
The number 2 raised to any positive integer power must be even (because it is divisible by 2) and the number 3 raised to any positive integer power must be odd (since none of its
Cases such as log10 2 can be treated similarly.
Types
An irrational number may be algebraic, that is a real root of a polynomial with integer coefficients. Those that are not algebraic are transcendental.
Algebraic
The real algebraic numbers are the real solutions of polynomial equations
where the coefficients are integers and . An example of an irrational algebraic number is x0 = (21/2 + 1)1/3. It is clearly algebraic since it is the root of an integer polynomial, (x3 − 1)2 = 2, which is equivalent to x6 − 2x3 − 1 = 0. This polynomial has no rational roots, since the rational root theorem shows that the only possibilities are ±1, but x0is greater than 1. So x0 is an irrational algebraic number. There are countably many algebraic numbers, since there are countably many integer polynomials.
Transcendental
So almost all irrational numbers are transcendental. Examples are e r and π r, which are transcendental for all nonzero rational r.
Because the algebraic numbers form a subfield of the real numbers, many irrational real numbers can be constructed by combining transcendental and algebraic numbers. For example, 3π + 2, π + √2 and e√3 are irrational (and even transcendental).
Decimal expansions
The decimal expansion of an irrational number never repeats (meaning the decimal expansion does not repeat the same number or sequence of numbers) or terminates (this means there is not a finite number of nonzero digits), unlike any rational number. The same is true for
To show this, suppose we divide integers n by m (where m is nonzero). When long division is applied to the division of n by m, there can never be a remainder greater than or equal to m. If 0 appears as a remainder, the decimal expansion terminates. If 0 never occurs, then the algorithm can run at most m − 1 steps without using any remainder more than once. After that, a remainder must recur, and then the decimal expansion repeats.
Conversely, suppose we are faced with a repeating decimal, we can prove that it is a fraction of two integers. For example, consider:
Here the repetend is 162 and the length of the repetend is 3. First, we multiply by an appropriate power of 10 to move the decimal point to the right so that it is just in front of a repetend. In this example we would multiply by 10 to obtain:
Now we multiply this equation by 10r where r is the length of the repetend. This has the effect of moving the decimal point to be in front of the "next" repetend. In our example, multiply by 103:
The result of the two multiplications gives two different expressions with exactly the same "decimal portion", that is, the tail end of 10,000A matches the tail end of 10A exactly. Here, both 10,000A and 10A have .162162162... after the decimal point.
Therefore, when we subtract the 10A equation from the 10,000A equation, the tail end of 10A cancels out the tail end of 10,000A leaving us with:
Then
is a ratio of integers and therefore a rational number.
Irrational powers
Dov Jarden gave a simple non-constructive proof that there exist two irrational numbers a and b, such that ab is rational:[28][29]
- Consider √2√2; if this is rational, then take a = b = √2. Otherwise, take a to be the irrational number √2√2 and b = √2. Then ab = (√2√2)√2 = √2√2·√2 = √22 = 2, which is rational.
Although the above argument does not decide between the two cases, the Gelfond–Schneider theorem shows that √2√2 is transcendental, hence irrational. This theorem states that if a and b are both algebraic numbers, and a is not equal to 0 or 1, and b is not a rational number, then any value of ab is a transcendental number (there can be more than one value if complex number exponentiation is used).
An example that provides a simple constructive proof is[30]
The base of the left side is irrational and the right side is rational, so one must prove that the exponent on the left side, , is irrational. This is so because, by the formula relating logarithms with different bases,
which we can assume, for the sake of establishing a contradiction, equals a ratio m/n of positive integers. Then hence hence hence , which is a contradictory pair of prime factorizations and hence violates the fundamental theorem of arithmetic (unique prime factorization).
A stronger result is the following:[31] Every rational number in the interval can be written either as aa for some irrational number a or as nn for some natural number n. Similarly,[31] every positive rational number can be written either as for some irrational number a or as for some natural number n.
Open questions
It is not known if (or ) is irrational. In fact, there is no pair of non-zero integers for which it is known whether is irrational. Moreover, it is not known if the set is algebraically independent over .
It is not known if
In constructive mathematics
In
Set of all irrationals
Since the reals form an
Under the usual (Euclidean) distance function , the real numbers are a metric space and hence also a topological space. Restricting the Euclidean distance function gives the irrationals the structure of a metric space. Since the subspace of irrationals is not closed, the induced metric is not
Furthermore, the set of all irrationals is a disconnected metrizable space. In fact, the irrationals equipped with the subspace topology have a basis of clopen groups so the space is zero-dimensional.
See also
- Brjuno number
- Computable number
- Diophantine approximation
- Proof that e is irrational
- Proof that π is irrational
- Square root of 3
- Square root of 5
- Trigonometric number
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References
- ^ The 15 Most Famous Transcendental Numbers. by Clifford A. Pickover. URL retrieved 24 October 2007.
- S2CID 123995083.
- ISBN 978-0-486-60045-1.
- S2CID 126296119.
- S2CID 115390951.
- ^ Kline, M. (1990). Mathematical Thought from Ancient to Modern Times, Vol. 1. New York: Oxford University Press (original work published 1972), p. 33.
- ^ Kline 1990, p. 32.
- ^ a b Kline 1990, p. 34.
- ^ Kline 1990, p. 48.
- ^ Kline 1990, p. 49.
- ^ Charles H. Edwards (1982). The historical development of the calculus. Springer.
- ^ Kline 1990, p. 50.
- S2CID 124565880..
- ^ Bag, Amulya Kumar (1990). "Ritual Geometry in India and its Parallelism in other Culture Areas". Indian Journal of History of Science. 25.
- ISBN 1-4020-0260-2..
- OCLC 414892.
It has been claimed also that the first recognition of incommensurables appears in India during the Sulbasutra period, but such claims are not well substantiated. The case for early Hindu awareness of incommensurable magnitudes is rendered most unlikely by the lack of evidence that Indian mathematicians of that period had come to grips with fundamental concepts.
- ^ Datta, Bibhutibhusan; Singh, Awadhesh Narayan (1993). "Surds in Hindu mathematics" (PDF). Indian Journal of History of Science. 28 (3): 253–264. Archived from the original (PDF) on 2018-10-03. Retrieved 18 September 2018.
- JSTOR 2691411.
- ^ O'Connor, John J.; Robertson, Edmund F. (1999). "Arabic mathematics: forgotten brilliance?". MacTutor History of Mathematics Archive. University of St Andrews..
- ^ S2CID 121416910. See in particular pp. 254 & 259–260.
- ISBN 1-4020-0260-2..
- ^ Cajori, Florian (1928). A History of Mathematical Notations (Vol.1). La Salle, Illinois: The Open Court Publishing Company. pg. 269.
- ^ (Cajori 1928, pg.89)
- ^ Salvatore Pincherle (1880). "Saggio di una introduzione alla teoria delle funzioni analitiche secondo i principii del prof. C. Weierstrass". Giornale di Matematiche: 178–254, 317–320.
- ^ Lambert, J. H. (1761). "Mémoire sur quelques propriétés remarquables des quantités transcendentes, circulaires et logarithmiques" (PDF). Mémoires de l'Académie royale des sciences de Berlin (in French): 265–322. Archived (PDF) from the original on 2016-04-28.
- S2CID 123203471.
- MR 1891736
- ^ Jarden, Dov (1953). "Curiosa No. 339: A simple proof that a power of an irrational number to an irrational exponent may be rational". Scripta Mathematica. 19: 229. copy
- ISBN 0-631-19544-0.
- ^ Lord, Nick, "Maths bite: irrational powers of irrational numbers can be rational", Mathematical Gazette 92, November 2008, p. 534.
- ^ Mathematical Gazette96, March 2012, pp. 106-109.
- ^ Albert, John. "Some unsolved problems in number theory" (PDF). Department of Mathematics, University of Oklahoma. (Senior Mathematics Seminar, Spring 2008 course)
- ISBN 978-1-470-45144-8.
- ISBN 0-387-15066-8.
Further reading
- Adrien-Marie Legendre, Éléments de Géometrie, Note IV, (1802), Paris
- Rolf Wallisser, "On Lambert's proof of the irrationality of π", in Algebraic Number Theory and Diophantine Analysis, Franz Halter-Koch and Robert F. Tichy, (2000), Walter de Gruyter
External links
- Zeno's Paradoxes and Incommensurability Archived 2016-05-13 at the Wayback Machine (n.d.). Retrieved April 1, 2008