This paper describes a procedure for the construction of monopoles on three-dimensional Euclidean space, starting from their rational maps. A companion paper, ‘Euclidean monopoles and rational maps’, to appear in the same journal, describes the assignment to a monopole of a rational map, from $\Bbb{CP}^1$ to a suitable flag manifold. In describing the reverse direction, this paper completes the proof of the main theorem therein.

A construction of monopoles from solutions to Nahm's equations (a system of ordinary differential equations) has been well-known for certain gauge groups for some time. These solutions are hard to construct however, and the equations themselves become increasingly unwieldy when the gauge group is not $\mbox{SU}(2).$

Here, in contrast, a rational map is the only initial data. But whereas one can be reasonably explicit in moving from Nahm data to a monopole, here the monopole is only obtained from the rational map after solving a partial differential equation.

A non-linear flow equation, essentially just the path of steepest descent down the Yang-Mills-Higgs functional, is set up. It is shown that, starting from an ‘approximate monopole’ - constructed explicitly from the rational map - a solution to the flow must exist, and converge to an exact monopole having the desired rational map.

1991 Mathematics Subject Classification: 53C07, 53C80, 58D27, 58E15, 58G11.

In this paper we use the Hecke algebra of type $\bf B$ to define a new algebra $\mathcal S$ which is an analogue of the $q$-Schur algebra. We show that $\mathcal S$ has ‘generic’ basis which is independent of the choice of ring and the parameters $q$ and $Q$. We then construct Weyl modules for $\mathcal S$ and obtain, as factor modules, a family of irreducible $\mathcal S$-modules defined over any field.

1991 Mathematics Subject Classification: 16G99, 20C20, 20G05.

The main part of the paper deals with local existence and global existence versus blow-up for solutions of the Laplace equation in bounded domains with a non-linear dynamical boundary condition. More precisely, we study the problem consisting in: (1) the Laplace equation in $(0, \infty) \times \Omega$; (2) a homogeneous Dirichlet condition $(0, \infty) \times \Gamma_0$; (3) the dynamical boundary condition $ \frac {\partial u}{\partial \nu} = - |u_t|^{m-2} u_t + |u|^{p - 2} u$ on $(0, \infty) \times \Gamma_1$; (4) the initial condition $u(0, x) = u_0 (x)$ on $\partial \Omega$. Here $\Omega$ is a regular and bounded domain in $\mathbb{R}^n$, with $n \ge 1$, and $\Gamma_0$ and $\Gamma_1$ endow a measurable partition of $\partial \Omega$. Moreover, $m>1$, $2 \le p < r$, where $r = 2 (n - 1) / (n - 2)$ when $n \ge 3$, $r = \infty$ when $n = 1,2$, and $u_0 \in H^{1/2} (\partial \Omega)$, $u_0 = 0$ on $\Gamma_0$.

The final part of the paper deals with a refinement of a global non-existence result by Levine, Park and Serrin, which is applied to the previous problem.

We derive absolute stability results of Popov-type for infinite-dimensional systems in an input-output setting. Our results apply to feedback systems where the linear part is the series interconnection of an $L^2$-stable linear system and an integrator, and the non-linearity satisfies a sector condition which allows for non-linearities with lower gain equal to zero (such as saturation, or more generally, bounded non-linearities). These results are used to prove convergence and stability properties of low-gain integral feedback control applied to $L^2$-stable linear systems subject to actuator and sensor non-linearities. The class of actuator/sensor non-linearities under consideration contains standard non-linearities which are important in control engineering such as saturation and deadzone. Moreover, we use the input-output theory developed to derive state-space results on absolute stability and low-gain integral control for strongly stable well-posed infinite-dimensional linear systems.

Via a random construction we establish necessary conditions for $L^p (\ell^q)$ inequalities for certain families of operators arising in harmonic analysis. In particular, we consider dilates of a convolution kernel with compactly supported Fourier transform, vector maximal functions acting on classes of entire functions of exponential type, and a characterization of Sobolev spaces by square functions and pointwise moduli of smoothness.

We study the arithmetic of certain del Pezzo surfaces of degree 2. We produce examples of Brauer-Manin obstruction to the Hasse principle, coming from 2- and 4-torsion elements in the Brauer group.

In an earlier paper (see Proc. London Math. Soc. (3) 84 (2002) 257–288) we showed that an irreducible integral binary cubic form $f(x, y)$ attains infinitely many prime values, providing that it has no fixed prime divisor. We now extend this result by showing that $f(m, n)$ still attains infinitely many prime values if $m$ and $n$ are restricted by arbitrary congruence conditions, providing that there is still no fixed prime divisor.

Two immediate consequences for the solvability of diagonal cubic Diophantine equations are given.

We describe the current state of progress on the maximal subgroup problem for the Monster sporadic simple group. Any unknown maximal subgroup is an almost simple group whose socle is in one of 19 specified isomorphism classes.

2000 Mathematical Subject Classification:20D08.

As is usual in prime number theory, write $$\psi(x,q,a)=\sum_{{n\le x}\atop{n\equiv a\pmod q}}\Lambda(n).$$ It is well known that when $q$ is close to $x$ the average value of $$V(x,q)=\sum_{{a=1}\atop{(a,q)=1}}^q \left|\psi(x,q,a)-{x\over{\phi(q)}}\right|^2$$ is about $x\log q$, and recently Friedlander and Goldston have shown that if $$U(x,q)=x\log q-x\left( \gamma+\log 2\pi+\sum_{p|q}\frac{\log p}{p-1} \right),$$ then the first moment of $V(x,q)-U(x,q)$ is small. In this memoir it is shown that the same is true for all moments. 2000 Mathematics Subject Classification: 11N13.

We investigate the existence of a weak solution $u$ to the quasilinear two-point boundary value problem

(P) $$ - (|u'|^{p-2} u')' = \lambda_k |u|^{p-2} u + f(x) , \quad 0 < x < a ; \quad u(0) = u(a) = 0.$$

We assume that $1 < p < \infty$, $p \not= 2$, $0 < a < \infty$, and that $f \in L^1(0,a)$ is a given function. The number $\lambda_k$ stands for the $k$-th eigenvalue of the one-dimensional $p$-Laplacian. Let $\sin_p(\pi_p x/a)$ denote the eigenfunction associated with $\lambda_1$; then $\sin_p(k \pi_p x/a)$ is the eigenfunction associated with $\lambda_k$. We show the existence of solutions to (P) in the following cases.

(i) When $k=1$ and $f$ satisfies the orthogonality condition

$$\int_0^a f(x) \sin_p(\pi_p x/a) \, dx = 0,$$

the set of solutions is bounded.

(ii) If $k=1$ and $f_t \in L^1(0,a)$ is a continuous family parametrized by $t \in [0,1]$, with

$$\int_0^a f_0(x) \sin_p(\pi_p x/a) \, dx < 0 < \int_0^a f_1(x) \sin_p(\pi_p x/a) \, dx,$$

then there exists some $t^* \in [0,1]$ such that (P) has a solution for $f = f_{t^*}$. Moreover, an appropriate choice of $t^*$ yields a solution $u$ with an arbitrarily large $L^1(0,a)$-norm which means that such $f$ cannot be orthogonal to $\sin_p(\pi_p x/a)$.

(iii) When $k \geq 2$ and $f$ satisfies a set of orthogonality conditions to $\sin_p(k \pi_p x/a)$ and/or $\cos_p(k \pi_p x/a)$ on the subintervals $[ma/k, (m+1)a/k]$ (with $m = 0, 1, \dots, k-1$) and/or $[(m - \frac12 )a/k, (m + \frac12)a/k]$ (with $m = 1, 2, \dots, k-1$), again, the set of solutions is bounded.

(iv) If $k \geq 2$ and $\{ f_t \in L^1(0,a) : t \in [0,1] \}$ is a continuous family satisfying either

$$\int_0^a f_0(x) \sin_p(k \pi_p x/a) \, dx < 0 < \int_0^a f_1(x) \sin_p(k \pi_p x/a) \, dx$$

or another related condition, then there exists some $t^* \in [0,1]$ such that (P) has a solution for $f = f_{t^*}$.

Prüfer's transformation plays the key role in our proofs.

2000 Mathematical Subject Classification: primary 34B16, 47J10; secondary 34L40, 47H30.

Let $F$ be a $p$-adic field and $K/F$ a finite, cyclic, tamely ramified field extension. We show, without any restriction, that base change from $F$ to K, in the sense of Arthur and Clozel, is compatible with the operation of $K/F$-lifting for simple characters defined by the authors in an earlier paper. This extends and completes the previous work, and represents a substantial step towards the goal of describing tamely ramified base change in terms of the structure theory of representations of Bushnell and Kutzko. We also give a consequence for the Langlands correspondence. The main step in the proof is to show that lifting of simple characters is compatible with automorphic induction, in the sense of Henniart and Herb, through an unramified extension. This is achieved via a new and independently interesting result concerning the conductor of a pair of supercuspidal representations. It gives a complete description of the minima of (a suitably normalized version of) the conductor when one of the representations is fixed. All results concerning the conductor are also valid for non-Archimedean local fields of positive characteristic.

The largest prime factor of $X^{3}+2$ was investigated in 1978 by Hooley, who gave a conditional proof that it is infinitely often at least as large as $X^{1+\delta}$, with a certain positive constant $\delta$. It is trivial to obtain such a result with $\delta=0$. One may think of Hooley's result as an approximation to the conjecture that $X^{3}+2$ is infinitely often prime. The condition required by Hooley, his R$^{*}$ conjecture, gives a non-trivial bound for short Ramanujan--Kloosterman sums. The present paper gives an unconditional proof that the largest prime factor of $X^{3}+2$ is infinitely often at least as large as $X^{1+\delta}$, though with a much smaller constant than that obtained by Hooley. In order to do this we prove a non-trivial bound for short Ramanujan--Kloosterman sums with smooth modulus. It is also necessary to modify the Chebychev method, as used by Hooley, so as to ensure that the sums that occur do indeed have a sufficiently smooth modulus. 2000 Mathematics Subject Classification: 11N32.

Let $R$ be a ring and $M_R$ be an $R$-module. We characterize the existence of ${\rm Add} M$-covers and ${\rm Add} M$-envelopes in terms of finiteness conditions of $M$ over its endomorphism ring. We then present some applications related to the existence of well-behaved direct sum decompositions for direct products of copies of $M$. Our results can be viewed as natural extensions of classical theorems of Bass and Chase on coherent and perfect rings.

A line bundle over a complex projective variety is called big and 1-ample if a large multiple of it is generated by global sections and a morphism induced by the evaluation of the spanning sections is generically finite and has at most 1-dimensional fibers. A vector bundle is called big and 1-ample if the relative hyperplane line bundle over its projectivisation is big and 1-ample.

The main theorem of the present paper asserts that any complex projective manifold of dimension 4 or more, whose tangent bundle is big and 1-ample, is equal either to a projective space or to a smooth quadric. Since big and 1-ample bundles are ‘almost’ ample, the present result is yet another extension of the celebrated Mori paper ‘Projective manifolds with ample tangent bundles’ (Ann. of Math. 110 (1979) 593–606).

The proof of the theorem applies results about contractions of complex symplectic manifolds and of manifolds whose tangent bundles are numerically effective. In the appendix we re-prove these results.

Hamiltonian stationary Lagrangian surfaces are Lagrangian surfaces in a four-dimensional Kähler manifold which are critical points of the area functional for Hamiltonian infinitesimal deformations. In this paper we analyze these surfaces in the complex projective plane: in a previous work we showed that they correspond locally to solutions to an integrable system, formulated as a zero curvature on a (twisted) loop group. Here we give an alternative formulation, using non-twisted loop groups and, as an application, we show in detail why Hamiltonian stationary Lagrangian tori are finite type solutions, and eventually describe the simplest of them: the homogeneous ones.

In this paper we determine the possible Hilbert functions of a Cohen--Macaulay local ring of dimension $d$ and multiplicity $e$, in the case where the embedding dimension $v$ satisfies $v=e+d-3$ and the Cohen--Macaulay type is less than or equal to $e-3$.

1991 Mathematics Subject Classification: primary 13D40; secondary 13P99.

In this paper we obtain optimal bounds for the length of the longest arithmetic progression in various kinds of sum-sets. As an application, we derive a sharp estimate for the number of sets $A$ of residues modulo a prime $n$ such that no subsum of $A$ equals $x$ modulo $n$, where $x$ is a fixed residue modulo $n$.

Let $A$ and $B$ be C*-algebras, with $A$ separable and $B$$\sigma$-unital and stable. It is shown that there are natural isomorphisms

$$ E(A,B) = KK(SA,Q(B)) = [SA,Q(B) \otimes K], $$

where $SA=C_0(0, 1) \otimes A$, $[\cdot, \cdot]$ denotes the set of homotopy classes of *-homomorphisms, $Q(B) = M(B) / B$ is the generalized Calkin algebra, and $K$ denotes the C*-algebra of compact operators of an infinite-dimensional separable Hilbert space.

A theorem of J. Hersch (1970) states that for any smooth metric on $S^2$, with total area equal to $4\pi$, the first non-zero eigenvalue of the Laplace operator acting on functions is less than or equal to 2 (this being the value for the standard round metric). For metrics invariant under the standard $S^1$-action on $S^2$, one can restrict the Laplace operator to the subspace of $S^1$-invariant functions and consider its spectrum there. The corresponding eigenvalues will be called invariant eigenvalues, and the purpose of this paper is to analyse their possible values.

We first show that there is no general analogue of Hersch's theorem, by exhibiting explicit families of $S^1$-invariant metrics with total area $4\pi$, where the first invariant eigenvalue ranges through any value between 0 and $\infty$. We then restrict ourselves to $S^1$-invariant metrics that can be embedded in $\mathbb{R}^3$ as surfaces of revolution. For this subclass we are able to provide optimal upper bounds for all invariant eigenvalues. As a consequence, we obtain an analogue of Hersch's theorem with an optimal upper bound (greater than 2 and geometrically interesting). This subclass of metrics on $S^2$ includes all $S^1$-invariant metrics with non-negative Gauss curvature.

One of the key ideas in the proofs of these results comes from symplectic geometry, and amounts to the use of the moment map of the $S^1$-action as a coordinate function on $S^2$.

2000 Mathematical Subject Classification: primary 35P15; secondary 58J50, 53D20.

The concept of generalized functions taking values in a differentiable manifold is extended to a functorial theory. We establish several characterization results which allow a global intrinsic formulation both of the theory of manifold-valued generalized functions and of generalized vector bundle homomorphisms. As a consequence, a characterization of equivalence that does not resort to derivatives (analogous to scalar-valued cases of Colombeau's construction) is achieved. These results are employed to derive a point value description of all types of generalized functions valued in manifolds and to show that composition can be carried out unrestrictedly. Finally, a new concept of association adapted to the present setting is introduced.