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DECEMBER 2003

**When Does Water Find the Shortest Path Downhill? The Geometry of Steepest Descent Curves**

by Daniel Drucker and Stephen A. Williams

[email protected] and [email protected]

Water flowing down a hill can reasonably be assumed to follow steepest descent curves. Such curves usually are not geodesics, but we determine exactly when they are. Surprisingly, there are surfaces all of whose steepest descent curves are geodesics; we classify them and explain how to construct them. We also determine when steepest descent curves are asymptotic or principal and give an example of a surface all of whose steepest descent curves are asymptotic.

**The Role of Logic in Teaching Proof**

by Susanna Epp

[email protected]

Even simple mathematical proofs and disproofs are more logically complex than most mathematicians realize. Research by mathematics educators and cognitive psychologists supports the claim that the logical reasoning abilities needed to discover them are not widely present in the population. Two hypotheses are proposed to explain some of the reasons why so many students have difficulty with proof and disproof: differences between mathematical language and the language of everyday discourse, and the kinds of shortcuts and simplifications that have been part of students’ previous mathematical instruction. The article describes research about whether instruction can help students develop formal reasoning skills and suggests that such instruction can be successful when done with appropriate parallel development of transfer skills. The final sections discuss at what point the principles of logic should be introduced and give a variety of suggestions about how to teach them.

**Negative Theorems in Approximation Theory**

by Allan Pinkus

[email protected]

We consider some negative results in approximation theory, what they tell us, and why they are important. The results we present all center about one theme, namely, that approximation processes are necessarily of limited capability. As approximation theory is concerned with the ability to approximate functions and processes by simpler and more easily calculated objects, it is important to know these limitations. We discuss three main categories of results, with a few elementary examples from each. The first category consists of inverse theorems, the second set of results is connected with limitations of linear processes, and the third group is concerned with *n*-width type results.

**Eudoxus Meets Cayley**

by Richard E. Chandler, Carl D. Meyer, and Nicholas J. Rose

[email protected], [email protected], [email protected]

This article presents an intriguing marriage of classical geometry and the modern theory of stochastic matrices for the purpose of analyzing the limiting properties of subtriangles and supertriangles. The (*p,q*)-subtriangle of triangle *ABC* is triangle *A'B'C'* in which *A', B'*, and *C' *divide the sides *AB, BC*, and *CA*, respectively, in the ratio *p*:*q*. The (*p,q*)2-subtriangle of *ABC* is the (*p,q*)-subtriangle of* A'B'C'*. An ancient result is that the (1,1)-subtriangle of *ABC* is similar to *ABC*. After proving that the (1,2)2-subtriangle of *ABC* is also similar to *ABC*, we address the question of whether the (1,*n*)*n*-subtriangle of *ABC* is similar to *ABC* for all natural numbers *n*. The answer to this question along with the analysis of limiting behavior of various iterated subtriangle procedures is accomplished by applying the theory of stochastic matrices, matrix exponentials, and the gamma function.

**A Localized Probabilistic Approach to Circuit Analysis**

by Frank Blume

[email protected]

The paper demonstrates that the standard method of analyzing resistor networks via systems of linear equations is equivalent to a probabilistic approach that translates resistances into transitional probabilities in such a way that electric currents in the circuit can be calculated from the unique invariant probability vector of the corresponding stochastic matrix.

**Notes**

**A Topological Puzzle**

by Inta Bertuccioni

[email protected]

**The Ascoli-Arzelà Theorem via Tychonoff’s Theorem**

by David C. Ullrich

[email protected]

**Klee’s Trigonometry Problem**

by P.L. Kannappan

[email protected]

**Green’s Theorem and the Fundamental Theorem of Algebra**

by Paul Loya

[email protected]

**Mean Curvature and Asymptotic Volume of Small Balls**

by Dominique Hulin and Marc Troyanov

[email protected], [email protected]

**Timing Is Everything: The French Connection**

C.W. Groetsch

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**Reviews**

**Weighing the Odds: A Course in Probability and Statistics**

by David Williams

Reviewed by Ed Waymire

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**Drawbridge Up: MathematicsÂ—A Cultural Anathema**

by Hans Magnus Enzensberger

Reviewed by Marion D. Cohen

[email protected]

**Classical and Quantum Computation**

by A. Yu. Kitaev, A. H. Shen, and M. N. Vyalyi

Reviewed by Daniel Gottesman

[email protected]

**Telegraphic Reviews**