This is a paper about upper bounds for Dirichlet's L-function, L(s, χ), on its critical line (s + s¯ = 1). It is to be assumed throughout that, unless otherwise stated, the Dirichlet character, χ, is periodic modulo a prime, r, and is not the principal character mod r. Our main theorem below shows that, if ε > 0, then

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(where A is an absolute constant), for 0 < α = (log r)/(log t) [les ] 2/753 − ε. Somewhat weaker bounds are obtained for other cases where 0 < α [les ] 11/180 − ε. Note that in [13] it was shown that, for 0 < α [les ] 2/57,

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Our main theorem is a corollary of the new bounds we prove for certain exponential sums, S, with a Dirichlet character factor:

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where M2 [les ] 2M and f(x) is a real function whose derivatives satisfy certain conditions restricting their size.

Given compact sets E and F in [Copf ]n (n [ges ] 1) related by F = q−1(E) where q is a polynomial map, we are interested in the general problem of comparing minimal polynomials for E with minimal polynomials for F. Let α be an n-multi-index of length d. We define the classes of polynomials ℙ(α) := zα + [Copf ]d−1[z] and [Pscr ](α) := {p: p(z) = zα + [sum ]β[pr ]αaβzβ} where [pr ] denotes the usual graded lexicographic order. Polynomials in ℙ(α) or in [Pscr ](α) of least deviation from zero on E (with respect to the supremum norm) are called minimal polynomials for E. We prove that if q is a simple (i.e. qˆi(z) = zmi) polynomial mapping of degree m and if p is minimal polynomial for E then p ∘ q is a minimal polynomial for F = q−1(E) and, using some algebraic machinery, we can also construct minimal polynomials for E from minimal polynomials for F. The result seems to be new even in the one-dimensional case.

The Young–Fibonacci graph [ ][ ] is an important example (along with the Young lattice) of differential posets studied by Fomin and Stanley. For every differential poset there is a distinguished central measure called the Plancherel measure. We study the Plancherel measure and the associated Markov chain, the Plancherel process, on the Young–Fibonacci graph.

We establish a law of large numbers which implies that the Plancherel measure cannot be represented as a nontrivial mixture of central measures, i.e. is ergodic. Our second result claims the convergence of the level distributions of the Plancherel measure to the GEM(½) probability law in the space of nonnegative series with unit sum, which is a particular example of distribution from the class of Residual Allocation Models.

In order to obtain the Plancherel process as an image of a sequence of independent uniformly distributed random variables, we establish a new version of the Robinson–Schensted type correspondence between permutations and pairs of paths in the Young–Fibonacci graph. This correspondence is used to demonstrate a recurrence property of the Plancherel process.

If a Quillen model category can be specified using a certain logical syntax (intuitively, ‘is algebraic/combinatorial enough’), so that it can be defined in any category of sheaves, then the satisfaction of Quillen's axioms over any site is a purely formal consequence of their being satisfied over the category of sets. Such data give rise to a functor from the category of topoi and geometric morphisms to Quillen model categories and Quillen adjunctions.

For a hyperbolic knot, the excellent component of the character curve is the one containing the complete hyperbolic structure on the complement of the knot. In this paper we explain a method to compute the excellent component of the character variety of periodic knots. We apply the method to those knots obtained as the preimage of one component of a 2-bridge link by a cyclic covering of S3 branched on the other component. We call these knots periodic knots with rational quotient. Among this class of knots are the ‘Turk's head knots’. Finally we give some invariants deduced from the excellent component of the character curve, such as the h-polynomial and the limit of hyperbolicity for all the periodic knots with rational quotient, up to 10 crossings, which are not 2-bridge or toroidal.

In 1996, Cromwell and Nutt [7] found an upper bound on the arc index which is related to the minimal crossing number and conjectured that the upper bound achieves the lowest possible index for alternating links.

CONJECTURE. Let L be any prime link. Then α(L) [les ] c(L)+2. Moreover this inequality is strict if and only if L is not alternating.

In this paper, we define a new diagram, called a wheel diagram, of a link and use it to prove this conjecture.

It is an interesting open question when Dehn surgery on a knot in the 3-sphere S3 can produce a lens space (see [10, 12]). Some studies have been made for special knots; in particular, the question is completely solved for torus knots [21] and satellite knots [3, 29, 31]. It is known that there are many examples of hyperbolic knots which admit Dehn surgeries yielding lens spaces. For example, Fintushel and Stern [8] have shown that 18- and 19-surgeries on the (−2, 3, 7)-pretzel knot give lens spaces L(18, 5) and L(19, 7), respectively. However, there seems to be no essential progress on hyperbolic knots. It might be a reason that some famous classes of hyperbolic knots, such as 2-bridge knots [26], alternating knots [5], admit no surgery yielding lens spaces.

In this paper we focus on the genera of knots to treat the present condition methodically and show that there is a constraint on the order of the fundamental group of the resulting lens space obtained by Dehn surgery on a hyperbolic knot. Also, this new standpoint enables us to present a conjecture concerning such a constraint, which holds for all known examples.

Let S be a smooth hypersurface in ℝn with surface area measure ds and Gaussian curvature κ(s). Define the convolution operator T by

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for suitable functions f on ℝn. We are interested in the Lp − Lq mapping properties of T. Write [Sscr ] for the type set of T, the set

formula here

It is well known (see, e.g. [O1]) that [Sscr ] is contained in the closed triangle [Tscr ] with vertices (0, 0), (1, 1) and (n/(n+1), 1/(n+1)). This paper is concerned with estimates of the form

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The estimate (1) is interesting because it serves as a weak substitute for the L(n+1)/n − Ln+1 boundedness of T. For example, if S is compact and (1) holds, then well-known arguments show that [Sscr ] differs from the full triangle [Tscr ] by at most the point (n/(n + 1), 1/(n + 1)). Our main result is a condition sufficient to imply (1). Its statement requires the following definition.

We give necessary and sufficient conditions for a conformal foliation locally generated by conformal vector fields to produce harmonic morphisms. Natural constructions of harmonic maps and morphisms are thus obtained. Also we obtain reducibility results for harmonic morphisms induced by (infinitesimal) conformal actions on Einstein manifolds.

Let h be a harmonic function on ℝn of suitably restricted growth. It is known that if h vanishes, or is bounded, on the lattice ℤn−1 × {0}, then the same is true on ℝn−1 × {0}. This paper presents sharp results which show that, if n [ges ] 3, then the same conclusions can be drawn even if information about h is missing on a substantial proportion of the lattice points. As corollaries we obtain uniqueness and Liouville-type theorems for harmonic, and also polyharmonic, functions which improve results by several authors.

(Volume 129 (2000), 263–275)

It is with very great regret that we record the death of one of the authors, Frank Peterson, shortly before the appearance of the paper. The Authors and Editor wish to express their great personal and professional respect for him.