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1. CJM 2009 (vol 61 pp. 205)

Marshall, M.
Representations of Non-Negative Polynomials, Degree Bounds and Applications to Optimization
Natural sufficient conditions for a polynomial to have a local minimum at a point are considered. These conditions tend to hold with probability $1$. It is shown that polynomials satisfying these conditions at each minimum point have nice presentations in terms of sums of squares. Applications are given to optimization on a compact set and also to global optimization. In many cases, there are degree bounds for such presentations. These bounds are of theoretical interest, but they appear to be too large to be of much practical use at present. In the final section, other more concrete degree bounds are obtained which ensure at least that the feasible set of solutions is not empty.

Categories:13J30, 12Y05, 13P99, 14P10, 90C22

2. CJM 2002 (vol 54 pp. 897)

Fortuny Ayuso, Pedro
The Valuative Theory of Foliations
This paper gives a characterization of valuations that follow the singular infinitely near points of plane vector fields, using the notion of L'H\^opital valuation, which generalizes a well known classical condition. With that tool, we give a valuative description of vector fields with infinite solutions, singularities with rational quotient of eigenvalues in its linear part, and polynomial vector fields with transcendental solutions, among other results.

Categories:12J20, 13F30, 16W60, 37F75, 34M25

3. CJM 2001 (vol 53 pp. 33)

Borwein, Peter; Choi, Kwok-Kwong Stephen
Merit Factors of Polynomials Formed by Jacobi Symbols
We give explicit formulas for the $L_4$ norm (or equivalently for the merit factors) of various sequences of polynomials related to the polynomials $$ f(z) := \sum_{n=0}^{N-1} \leg{n}{N} z^n. $$ and $$ f_t(z) = \sum_{n=0}^{N-1} \leg{n+t}{N} z^n. $$ where $(\frac{\cdot}{N})$ is the Jacobi symbol. Two cases of particular interest are when $N = pq$ is a product of two primes and $p = q+2$ or $p = q+4$. This extends work of H{\o}holdt, Jensen and Jensen and of the authors. This study arises from a number of conjectures of Erd\H{o}s, Littlewood and others that concern the norms of polynomials with $-1,1$ coefficients on the disc. The current best examples are of the above form when $N$ is prime and it is natural to see what happens for composite~$N$.

Keywords:Character polynomial, Class Number, $-1,1$ coefficients, Merit factor, Fekete polynomials, Turyn Polynomials, Littlewood polynomials, Twin Primes, Jacobi Symbols
Categories:11J54, 11B83, 12-04

4. CJM 2000 (vol 52 pp. 1018)

Reichstein, Zinovy; Youssin, Boris
Essential Dimensions of Algebraic Groups and a Resolution Theorem for $G$-Varieties
Let $G$ be an algebraic group and let $X$ be a generically free $G$-variety. We show that $X$ can be transformed, by a sequence of blowups with smooth $G$-equivariant centers, into a $G$-variety $X'$ with the following property the stabilizer of every point of $X'$ is isomorphic to a semidirect product $U \sdp A$ of a unipotent group $U$ and a diagonalizable group $A$. As an application of this result, we prove new lower bounds on essential dimensions of some algebraic groups. We also show that certain polynomials in one variable cannot be simplified by a Tschirnhaus transformation.

Categories:14L30, 14E15, 14E05, 12E05, 20G10

5. CJM 2000 (vol 52 pp. 833)

Mináč, Ján; Smith, Tara L.
W-Groups under Quadratic Extensions of Fields
To each field $F$ of characteristic not $2$, one can associate a certain Galois group $\G_F$, the so-called W-group of $F$, which carries essentially the same information as the Witt ring $W(F)$ of $F$. In this paper we investigate the connection between $\wg$ and $\G_{F(\sqrt{a})}$, where $F(\sqrt{a})$ is a proper quadratic extension of $F$. We obtain a precise description in the case when $F$ is a pythagorean formally real field and $a = -1$, and show that the W-group of a proper field extension $K/F$ is a subgroup of the W-group of $F$ if and only if $F$ is a formally real pythagorean field and $K = F(\sqrt{-1})$. This theorem can be viewed as an analogue of the classical Artin-Schreier's theorem describing fields fixed by finite subgroups of absolute Galois groups. We also obtain precise results in the case when $a$ is a double-rigid element in $F$. Some of these results carry over to the general setting.

Categories:11E81, 12D15

6. CJM 1999 (vol 51 pp. 69)

Reichstein, Zinovy
On a Theorem of Hermite and Joubert
A classical theorem of Hermite and Joubert asserts that any field extension of degree $n=5$ or $6$ is generated by an element whose minimal polynomial is of the form $\lambda^n + c_1 \lambda^{n-1} + \cdots + c_{n-1} \lambda + c_n$ with $c_1=c_3=0$. We show that this theorem fails for $n=3^m$ or $3^m + 3^l$ (and more generally, for $n = p^m$ or $p^m + p^l$, if 3 is replaced by another prime $p$), where $m > l \geq 0$. We also prove a similar result for division algebras and use it to study the structure of the universal division algebra $\UD (n)$. We also prove a similar result for division algebras and use it to study the structure of the universal division algebra $\UD(n)$.

Categories:12E05, 16K20

7. CJM 1999 (vol 51 pp. 10)

Chacron, M.; Tignol, J.-P.; Wadsworth, A. R.
Tractable Fields
A field $F$ is said to be tractable when a condition described below on the simultaneous representation of quaternion algebras holds over $F$. It is shown that a global field $F$ is tractable i{f}f $F$ has at most one dyadic place. Several other examples of tractable and nontractable fields are given.

Categories:12E15, 11R52

8. CJM 1998 (vol 50 pp. 1189)

Engler, Antonio José
Totally real rigid elements and Galois theory
Abelian closed subgroups of the Galois group of the pythagorean closure of a formally real field are described by means of the inertia group of suitable valuation rings.

Categories:12F10, 12J20

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