The diffraction of a line source by an absorbing semi-infinite plane in the presence of a subsonic fluid flow is examined. Expressions for the total far field for the leading edge (no wake present) and the trailing edge (wake present) situation are given.

It is found that the presence of fluid flow attenuates the sound level in the shadow region of the absorbent screen. There is greater attenuation of the sound level in the trailing edge situation than there is for the leading edge situation.

Sound level attenuation on the source side of the absorbent screen is found to depend more critically on the absorptive properties of the screen than on the fluid flow. The radiated sound intensity, in the half space in which the source is located, can be considerably reduced by a suitable choice of the absorption parameter.

An integral operator is constructed which maps solutions of the reduced wave equation defined in exterior domains onto solutions of ∆n u+λ2(l+B(r))u = 0 (*) defined in exterior domains, where B(r) is a continuously differentiable function of compact support. This operator is then used to construct a solution to the exterior Neumann problem for (*) satisfying the Sommerfeld radiation condition at infinity. Such problems arise in connection with the scattering of acoustic waves in a non-homogeneous medium, and this paper gives a method for solving these problems which is suitable for analytic and numerical approximations.

Let A be a closed linear transformation from a real Hilbert space ℋ, with symmetric inner product 〈, 〉, into itself; and let f ∈ ℋ be given such that the problem Aø = f has a solution ø ∈ D(A), the domain of A. Then bivariational upper and lower bounds on 〈g, ø〉 for any g ∈ ℋ are exhibited when there exists a positive constant a such that 〈AΦ, AΦ⊖ ≧ a2〈Φ, Φ〉 for all Φ ∈ D(A). The applicability of the theory both to Fredholm integral equations and also to time-dependent diffusion equations is demonstrated.

Some characterisations of commutatitivity for C*-algebras are given in terms of inequalities involving sums and products of self-adjoint elements, and optimal constants are obtained for the corresponding inequalities for non-commutative C*-algebras.

This paper extends some recent results of V. Barbu and H. Brézis. It is concerned with bounded solutions of the problem pu″+qu′ ∈ Au, u(0) = a, where A is a maximal monotone operator in a real Hilbert space H and p and q are real functions. Existence and uniqueness theorems are proved, with results on integrability of solutions in various measure spaces on R+. T(t) denotes the family of contractions of D(A) generated by the equation and we obtain a regularising effect on the initial data. Some properties of this family of contractions are studied.

A large class of self-adjoint fourth-order differential equations has the property that if one solution is oscillatory then all solutions are oscillatory. This paper establishes necessary and sufficient conditions for this property to hold for a corresponding class of non-self-adjoint differential equations.

The theorems in this paper deal with the analogues for integrals of Hardy's Series Inequality and its generalisations.

It is shown that classical solutions backward in time to the alternatives to the Navier-Stokes equations proposed by Ladyzhenskaya cannot exist for all time. Estimates are obtained for the maximum interval of existence.

In a certain factor analysis model sampling formulae for maximum likelihood estimators of the parameters are found by inverting an augmented information matrix. The formulae were derived previously by other methods; but the techniques here employed may be of some general interest and applicable to other problems.