The Geometric Mean, Matrices, Metrics, and More
by Jimmie D. Lawson and Yongdo Lim
Beginning with the modest concept of the geometric mean of two positive numbers (the square root of their product), we give eight alternative characterizations, all of which have direct generalizations to positive definite matrices. These generalizations, though elementary in nature, touch on an interesting variety of modern mathematics: matrix and operator equations and inequalities, Bruhat-Tits metrics, weakly symmetric Riemannian spaces, Cartan-Hadamard manifolds, symplectic geometry, Riccati differential equations and related dynamical systems, convex programming, and symmetric cones and Jordan algebras.
Keplers Laws, Newtons Laws, and the Search for New Planets
by Robert Osserman
One of the major astronomical stories of the past decade has been the discovery of a series of planets orbiting other stars. None of these planets has actually been seen, but their presence is detected by the wobble they induce on the accompanying star. Even more impressive is the amount of detailed information about the unseen planets that can be derived, simply by using Newtons Laws applied to the motion of the star. This article explains how it is done, with particular emphasis on the minor changes that would have to be made to the standard treatment of Newtons Laws in calculus courses in order to give students access to this exciting new area of astronomical research.
Polynomial Equations and Circulant Matrices
by Dan Kalman and James E. White
firstname.lastname@example.org and email@example.com
Eigenvalues of circulant matrices provide a route to the zeroes of quartic polynomials. In fact, circulant matrices provide a unified approach to quadratic, cubic, and quartic equations. This article shows how special matrix algebras can provide information about polynomial equations and their roots. In addition to circulant matrices, which lead to Cardanos solution of the cubic, read about Klein matrices and Cartesian matrices. Klein matrices lead to Eulers solution of the quartic, while Cartesian matrices lead to a solution nearly identical to that of Descartes. An interactive graphical exploration of some related ideas is available on-line at the New Mathwright Library (www.mathwright.com). Look for the Mathwright workbook Cubic Equations (www.mathwright.com/book_pgs/book241.html), or for the Lava workbook Cardano (www.mathwright.com/lavapage.htm) (viewable on a webpage).
The Evolution of
Weierstrass, Luzin, and Intuition
by Jeremy Gray
The Sum of Some Convergent Series
by Ákos László
A Simple Proof of the Fredholm Alternative and a Characterization of the Fredholm Operators.
by A.G. Ramm
An Elementary Product Identity in Polynomial Dynamics
by Robert L. Benedetto
The Emergence of the Theory of Lie Groups: An Essay in the History of Mathematics (1869-1926)
By Thomas Hawkins
Reviewed by Armand Borel
By Peter D. Lax
Reviewed by Mark A. Kon