Brahmagupta's Identity
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In algebra, Brahmagupta's identity says that, for given n, the product of two numbers of the form a^2+nb^2 is itself a number of that form. In other words, the set of such numbers is
closed Closed may refer to: Mathematics * Closure (mathematics), a set, along with operations, for which applying those operations on members always results in a member of the set * Closed set, a set which contains all its limit points * Closed interval, ...
under multiplication. Specifically: :\begin \left(a^2 + nb^2\right)\left(c^2 + nd^2\right) & = \left(ac-nbd\right)^2 + n\left(ad+bc\right)^2 & & & (1) \\ & = \left(ac+nbd\right)^2 + n\left(ad-bc\right)^2, & & & (2) \end Both (1) and (2) can be verified by expanding each side of the equation. Also, (2) can be obtained from (1), or (1) from (2), by changing ''b'' to −''b''. This identity holds in both the
ring of integers In mathematics, the ring of integers of an algebraic number field K is the ring of all algebraic integers contained in K. An algebraic integer is a root of a monic polynomial with integer coefficients: x^n+c_x^+\cdots+c_0. This ring is often deno ...
and the ring of rational numbers, and more generally in any
commutative ring In mathematics, a commutative ring is a ring in which the multiplication operation is commutative. The study of commutative rings is called commutative algebra. Complementarily, noncommutative algebra is the study of ring properties that are not sp ...
.


History

The identity is a generalization of the so-called Fibonacci identity (where ''n''=1) which is actually found in
Diophantus Diophantus of Alexandria ( grc, Διόφαντος ὁ Ἀλεξανδρεύς; born probably sometime between AD 200 and 214; died around the age of 84, probably sometime between AD 284 and 298) was an Alexandrian mathematician, who was the aut ...
' ''
Arithmetica ''Arithmetica'' ( grc-gre, Ἀριθμητικά) is an Ancient Greek text on mathematics written by the mathematician Diophantus () in the 3rd century AD. It is a collection of 130 algebraic problems giving numerical solutions of determinate e ...
'' (III, 19). That identity was rediscovered by
Brahmagupta Brahmagupta ( – ) was an Indian mathematician and astronomer. He is the author of two early works on mathematics and astronomy: the ''Brāhmasphuṭasiddhānta'' (BSS, "correctly established doctrine of Brahma", dated 628), a theoretical trea ...
(598–668), an Indian mathematician and astronomer, who generalized it and used it in his study of what is now called Pell's equation. His '' Brahmasphutasiddhanta'' was translated from Sanskrit into Arabic by
Mohammad al-Fazari Muhammad ( ar, مُحَمَّد;  570 – 8 June 632 CE) was an Arab religious, social, and political leader and the founder of Islam. According to Islamic doctrine, he was a prophet divinely inspired to preach and confirm the monothe ...
, and was subsequently translated into Latin in 1126.George G. Joseph (2000). ''The Crest of the Peacock'', p. 306. Princeton University Press. . The identity later appeared in Fibonacci's '' Book of Squares'' in 1225.


Application to Pell's equation

In its original context, Brahmagupta applied his discovery to the solution of what was later called Pell's equation, namely ''x''2 − ''Ny''2 = 1. Using the identity in the form :(x_1^2 - Ny_1^2)(x_2^2 - Ny_2^2) = (x_1x_2 + Ny_1y_2)^2 - N(x_1y_2 + x_2y_1)^2, he was able to "compose" triples (''x''1, ''y''1, ''k''1) and (''x''2, ''y''2, ''k''2) that were solutions of ''x''2 − ''Ny''2 = ''k'', to generate the new triple :(x_1x_2 + Ny_1y_2 \,,\, x_1y_2 + x_2y_1 \,,\, k_1k_2). Not only did this give a way to generate infinitely many solutions to ''x''2 − ''Ny''2 = 1 starting with one solution, but also, by dividing such a composition by ''k''1''k''2, integer or "nearly integer" solutions could often be obtained. The general method for solving the Pell equation given by Bhaskara II in 1150, namely the chakravala (cyclic) method, was also based on this identity.


See also

* Brahmagupta matrix *
Brahmagupta polynomials Brahmagupta polynomials are a class of polynomials associated with the Brahmagupa matrix which in turn is associated with the Brahmagupta's identity. The concept and terminology were introduced by E. R. Suryanarayan, University of Rhode Island, Kin ...
* Brahmagupta–Fibonacci identity * Brahmagupta's interpolation formula *
Gauss composition law In mathematics, in number theory, Gauss composition law is a rule, invented by Carl Friedrich Gauss, for performing a binary operation on integral binary quadratic forms (IBQFs). Gauss presented this rule in his ''Disquisitiones Arithmeticae'', a ...
*
Indian mathematics Indian mathematics emerged in the Indian subcontinent from 1200 BCE until the end of the 18th century. In the classical period of Indian mathematics (400 CE to 1200 CE), important contributions were made by scholars like Aryabhata, Brahmagupta ...
* List of Indian mathematicians


References


External links


Brahmagupta's identity
at PlanetMath
Brahmagupta Identity
on MathWorld
A Collection of Algebraic Identities
{{Webarchive, url=https://web.archive.org/web/20120306122543/http://sites.google.com/site/tpiezas/005b , date=2012-03-06 Algebraic identities Brahmagupta