The AGM is defined as the limit of the interdependent sequences and . Assuming , we write:These two sequences converge to the same number, the arithmetic–geometric mean of x and y; it is denoted by M(x, y), or sometimes by agm(x, y) or AGM(x, y).
The arithmetic–geometric mean can be extended to complex numbers and, when the branches of the square root are allowed to be taken inconsistently, generally it is a multivalued function.[1]
Example
To find the arithmetic–geometric mean of a0 = 24 and g0 = 6, iterate as follows:The first five iterations give the following values:
The number of digits in which an and gn agree (underlined) approximately doubles with each iteration. The arithmetic–geometric mean of 24 and 6 is the common limit of these two sequences, which is approximately 13.4581714817256154207668131569743992430538388544.[2]
History
The first algorithm based on this sequence pair appeared in the works of Lagrange. Its properties were further analyzed by Gauss.[1]
Properties
Both the geometric mean and arithmetic mean of two positive numbers x and y are between the two numbers. (They are strictly between when x ≠ y.) The geometric mean of two positive numbers is never greater than the arithmetic mean.[3] So the geometric means are an increasing sequence g0 ≤ g1 ≤ g2 ≤ ...; the arithmetic means are a decreasing sequence a0 ≥ a1 ≥ a2 ≥ ...; and gn ≤ M(x, y) ≤ an for any n. These are strict inequalities if x ≠ y.
M(x, y) is thus a number between x and y; it is also between the geometric and arithmetic mean of x and y.
If r ≥ 0 then M(rx, ry) = r M(x, y).
There is an integral-form expression for M(x, y):[4]where K(k) is the complete elliptic integral of the first kind:Since the arithmetic–geometric process converges so quickly, it provides an efficient way to compute elliptic integrals, which are used, for example, in elliptic filter design.[5]
The arithmetic–geometric mean is connected to the Jacobi theta function by[6]which upon setting gives
That is to say that this quarter period may be efficiently computed through the AGM,
Other applications
Using this property of the AGM along with the ascending transformations of John Landen,[16]Richard P. Brent[17] suggested the first AGM algorithms for the fast evaluation of elementary transcendental functions (ex, cos x, sin x). Subsequently, many authors went on to study the use of the AGM algorithms.[18]
^Bullen, P. S. (2003). "The Arithmetic, Geometric and Harmonic Means". Handbook of Means and Their Inequalities. Dordrecht: Springer Netherlands. pp. 60–174. doi:10.1007/978-94-017-0399-4_2. ISBN 978-90-481-6383-0. Retrieved 2023-12-11.
^Dimopoulos, Hercules G. (2011). Analog Electronic Filters: Theory, Design and Synthesis. Springer. pp. 147–155. ISBN 978-94-007-2189-0.
^Borwein, Jonathan M.; Borwein, Peter B. (1987). Pi and the AGM: A Study in Analytic Number Theory and Computational Complexity (First ed.). Wiley-Interscience. ISBN 0-471-83138-7. pages 35, 40
^Schneider, Theodor (1941). "Zur Theorie der Abelschen Funktionen und Integrale". Journal für die reine und angewandte Mathematik. 183 (19): 110–128. doi:10.1515/crll.1941.183.110. S2CID 118624331.
^Todd, John (1975). "The Lemniscate Constants". Communications of the ACM. 18 (1): 14–19. doi:10.1145/360569.360580. S2CID 85873.
^G. V. Choodnovsky: Algebraic independence of constants connected with the functions of analysis, Notices of the AMS 22, 1975, p. A-486
^G. V. Chudnovsky: Contributions to The Theory of Transcendental Numbers, American Mathematical Society, 1984, p. 6
^Borwein, Jonathan M.; Borwein, Peter B. (1987). Pi and the AGM: A Study in Analytic Number Theory and Computational Complexity (First ed.). Wiley-Interscience. ISBN 0-471-83138-7. p. 45
^Newman, D. J. (1985). "A simplified version of the fast algorithms of Brent and Salamin". Mathematics of Computation. 44 (169): 207–210. doi:10.2307/2007804. JSTOR 2007804.
^Landen, John (1775). "An investigation of a general theorem for finding the length of any arc of any conic hyperbola, by means of two elliptic arcs, with some other new and useful theorems deduced therefrom". Philosophical Transactions of the Royal Society. 65: 283–289. doi:10.1098/rstl.1775.0028. S2CID 186208828.
^Brent, Richard P. (1976). "Fast Multiple-Precision Evaluation of Elementary Functions". Journal of the ACM. 23 (2): 242–251. CiteSeerX10.1.1.98.4721. doi:10.1145/321941.321944. MR 0395314. S2CID 6761843.
Daróczy, Zoltán; Páles, Zsolt (2002). "Gauss-composition of means and the solution of the Matkowski–Suto problem". Publicationes Mathematicae Debrecen. 61 (1–2): 157–218. doi:10.5486/PMD.2002.2713.