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American Mathematical Monthly - March 1999

 

MARCH 1999

n-Ellipses and the Minimum Distance Sum Problem
by Junpei Sekino
sekino@willamette.edu
Given n points in a plane, which we call foci, an n-ellipse is the locus of points in the plane the sum of whose distances from the foci is a constant. Like circles (1-ellipses) and ellipses (2-ellipses), n-ellipses are Jordan curves with convex interiors but come in a greater variety of shapes such as cross-sections of eggs and filberts; some of them even resemble contours of human faces, symmetric or asymmetric. After illustrating some of these figures (computer generated), we turn our attention to the critical points of the distance sum function whose contours are n-ellipses. We show that a critical point is nondegenerate if and only if the foci are noncollinear, and demonstrate that a nondegenerate critical point not only yields a unique point minimizing the distance sum function but also possesses a geometrically appealing property. The paper concludes with an optimization problem and a suggestion for a further generalization of an ellipse to an n-ellipse with weights.

The Bieberbach Conjecture and Milin's Functionals
by Arcadii Z. Grinshpan
azg@math.usf.edu
The Bieberbach conjecture (1916) for the Taylor coefficients of one-to-one analytic functions is one of the most famous and inspirational problems of mathematics. This conjecture remained open for nearly 70 years until in 1984 L. de Branges proved a stronger conjecture for certain logarithmic functionals proposed by I. M. Milin in 1971.

de Branges' approach was based on Loewner's parametric method (1923), a classic tool in geometric function theory, but it involved other fields of mathematics as well. Simplifications made by several authors during 1984-1997 have led to the present proof, which is now accessible to many readers.

 

Decision Making: A Golden Rule
by Dimitris A. Sardelis and Theodoros M. Valahas
dereef@hol.gr
This article is an educational exposition of a classical optimal-stopping problem, the dowry problem. Its solution is presented by building on easily understandable special cases. A pattern of optimal strategies emerges, which is ultimately expressed by general conditions that lead to the (1/e) golden decision rule.

 

A Tale of Two Integrals
by Vilmos Totik
totik@math.usf.edu
In the paper several approaches are presented to the following simple-looking but highly non-trivial combinatorial-analysis problem: [0,1] whose integrals ove [0,1] are both equal to 1. Show that there is some interval I [0,1] such that the integrals of f and g over I are both equal to 1/2. This problem is equivalent to the following problem without integrals: On a blackjack machine one can win or lose one dollar at a time. Suppose two players playing once per minute during a period find that eventually both of them win exactly 2N dollars. Show that there was a time interval during which both of them won exactly N dollars.

Six solutions are presented that are based on other combinatorial or geometrical/topological results. These are:

 

  • the Borsuk-Ulam antipodal theorem on continuous mappings of spheres;
  • the mountain climbing problem on two climbers staying always at the same altitude;
  • the chord theorem for parallel chords of planar curves;
  • the chess king-moving theorem on king walks on cells of the same colour;
  • the winding number theorem for vector fields;
  • the Jordan curve theorem on components of simple curves.

 

A Mathematical Excursion: From the Three-Door Problem to a Cantor-Type Set
by Jaume Paradís, Pelegrí Viader, and Lluís Bibiloni
jaume.paradis@econ.upf.es, pelegri.viader@econ.upf.es, l.bibiloni@uab.es
Can you imagine a connection between the well-known three-door problem--also known as the Monty Hall problem--and a set with a complex structure like Cantor's set? We offer a generalization of the three-door problem, the n-box problem, whose solution provides us with a system for real number representation, Pierce expansions. Through these expansions we get a new enumeration of the positive rationals and also a Cantor-type set containing only transcendental numbers.

 

NOTES

An Inequality Relating the Circumradius and Diameter of
Two-Dimensional Lattice-Point-Free Convex Bodies

by Poh Wah Awyong
awyongpw@nievax.nie.ac.sg

Cutting a Polyomino into Triangles of Equal Areas
by Sherman K. Stein
stein@math.ucdavis.edu

A Short Proof of Turaacute;nÕs Theorem
by William Staton
mmstaton@olemiss.edu

A Characterization of the Set of Points of Continuity of a Real Function
by Sung Soo Kim
sskim@newton.hanyang.ac.kr

THE EVOLUTION OF...
The Literal Calculus of Vieacute;te and Descartes

by I. G. Bashmakova and G. S. Smirnova

PROBLEMS AND SOLUTIONS

REVIEWS

An Accompaniment to Higher Mathematics
By George R. Exner

Journey Into Mathemtics: An Introduction to Proofs
By Joseph Rotman

Mathematical Thinking: Problem Solving and Proofs
By John P. D'Angelo and Douglas B. West
Reviewed by Joseph H. Silverman
jhs@math.brown.edu

The Social Life of Numbers: A Quechua Ontology of Numbers
and Philosophy of Arithmetic

By Gary Urton

Reviewed by John Meier and Trisha Thorme
meierj@lafayette.edu, tat2@cornell.edu

TELEGRAPHIC REVIEWS

EDITOR'S ENDNOTES

 

 

Dummy View - NOT TO BE DELETED