On Euclid’s algorithm and elementary number theory

Algorithms can be used to prove and to discover new theorems. This paper shows how algorithmic skills in general, and the notion of invariance in particular, can be used to derive many results from Euclid’s algorithm. We illustrate how to use the algorithm as a verification interface (i.e., how to v...

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Bibliographic Details
Published in:Science of computer programming Vol. 76; no. 3; pp. 160 - 180
Main Authors: Backhouse, Roland, Ferreira, João F.
Format: Journal Article
Language:English
Published: Elsevier B.V 01.03.2011
Subjects:
Algorithm derivation
Calculational method
Eisenstein array
Eisenstein–Stern tree (aka Calkin–Wilf tree)
Enumeration algorithm
Euclid’s algorithm
Greatest common divisor
Invariant
Number theory
Rational number
Stern–Brocot tree
Stern–Brocot tree
Invariant
Eisenstein array
Enumeration algorithm
Calculational method
Euclid’s algorithm
Rational number
Greatest common divisor
Eisenstein–Stern tree (aka Calkin–Wilf tree)
Number theory
Algorithm derivation
ISSN:0167-6423, 1872-7964
Online Access:Get full text
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Summary:Algorithms can be used to prove and to discover new theorems. This paper shows how algorithmic skills in general, and the notion of invariance in particular, can be used to derive many results from Euclid’s algorithm. We illustrate how to use the algorithm as a verification interface (i.e., how to verify theorems) and as a construction interface (i.e., how to investigate and derive new theorems). The theorems that we verify are well-known and most of them are included in standard number-theory books. The new results concern distributivity properties of the greatest common divisor and a new algorithm for efficiently enumerating the positive rationals in two different ways. One way is known and is due to Moshe Newman. The second is new and corresponds to a deforestation of the Stern–Brocot tree of rationals. We show that both enumerations stem from the same simple algorithm. In this way, we construct a Stern–Brocot enumeration algorithm with the same time and space complexity as Newman’s algorithm. A short review of the original papers by Stern and Brocot is also included.
ISSN:0167-6423
1872-7964
DOI:10.1016/j.scico.2010.05.006