In this paper we prove

Theorem 1. If N is a natural number and I is an interval of N consecutive integers, then there is a 1–1 correspondencef: {1, 2,…, N} → I such that (i, f(i)) = 1 for 1 ≤ i ≤ N.

When K is a convex body in d-dimensional euclidean space E and 0 ≤ s ≤ d, the s-skeleton of K, denoted skel(s)K, consists of those points of K which are not centres of (s + l)-dimensional balls contained in K. The s-skeleton is thus the union of the extreme faces of K having dimension at most s. The s-skeleton is a -set [see 6] and it is therefore measurable with respect to the s-dimensional Hausdorff measure ℋ(s) [see 7]; here we normalize ℋ(s) so that it assigns unit measure to the s-dimensional unit cube.

The first problem in diophantine approximation is for a given real number ξ, and positive number x to find a fraction s/t with t ≤ x which is close to ξ. This problem can be rephrased in geometric terms. Given a vector v in ℝ2, find a vector a = (a1, a2) with integer coordinates and 1 1 ≤ al ≥ x such that the vectors a and v are nearly parallel. Simultaneous approximation of d − 1 real numbers can be recast in terms of approximation of the angle between two vectors in d-dimensional Euclidean space.

Unfortunately the contours used in the proof of Lemma 3 of my recent paper «this volume pp. 62–71» have not all been specified in the appropriate way. The contours used at the foot of page 66 should be

and middle contour used near the middle of page 67 should be

Let X be a compact metric space. By an invariant measure on X we shall mean a finitely additive non-negative real-valued function μ. on the Borel σ-algebra in X, such that μ(X) = 1 and Borel subsets of X have equal measure if there exists an isometry from one onto the other (not necessarily extendable to the whole of X). We note in passing that an isometric copy of a Borel set is necessarily itself Borel, by a well-known theorem of Souslin (see [3], §39. V). In Problem 2 of the Scottish Book (17. VII. 1935; see [2]), Banach and Ulam asked whether every non-empty compact metric space admits an invariant measure. This problem remains open; we give a positive answer in a veryspecial case.

The temporal and spatial linear instability of Poiseuille flow through pipes of arbitrary cross-section is discussed for large Reynolds numbers (R). For a pipe whose aspect ratio is finite, neutral stability (lower branch) is found to be governed by disturbance modes of large axial wavelength (of order hR, where h is a characteristic cross-sectional dimension). By contrast, spatial instability for finite aspect ratios is governed by length scales between O(h) and O(hR). When the aspect ratio is increased to O(R1/7), however, these two characteristic length scales both become O(R1/7h) and a match with plane channel flow instability is achieved. Thus the general cross-section produces temporal and spatial instability if the aspect ratio is O(R1/7). Further, in the flow in a rectangular pipe neutral stability (lower branch) exists for some finite aspect ratios, while for the flow in any non-circular elliptical pipe spatial instability is possible. It is suggested that both temporal and spatial instability occur for a wide range of pipe cross-sections of finite aspect ratio. Part 2 (Smith 1979a), which studies the upper branch neutral stability, confirms the importance of the O(hR) scale modes in neutral stability for finite aspect ratios.

Before stating the main theorem, we would like to recall the basic properties of the “zeta-functions” attached to cusp forms on SL (2, ℤ). Let k be an even integer ≥ 12 and f a cusp form of weight k on SL (2, ℤ) with q-expansion We shall assume that c1 = 1, and that f is an eigenfunction of the Hecke operators. Define φ(s) as the Dirichlet series The series and the product

over the primes are equal and absolutely convergent for Re (S) > ½(k + 1).

A higher-order, double boundary-layer theory is employed to investigate the mass transport velocity due to two-dimensional standing waves in a system consisting of two semi-infinite, homogeneous fluids of different densities and viscosities. For moderately large wave amplitudes, the leading correction to the tangential mass transport velocity near the interface is extremely significant and may typically contribute about 20% of the total velocity.

A natural and welcomed decomposition theorem for elements in the positive cone of the tensor product of Archimedean vector lattices leads to substantial simplifications in the theory of tensor products of Archimedean vector lattices.

This second part follows on directly from the first part that appears on pages 125–156 of this volume. The references for this part are included amongst the references appearing at the end of the first part.

We denote by S the unit sphere in ℝ3, and µ is the rotationally invariant measure, generalizing surface area on S; thus µS = 4π. We identify directions (or unit vectors) in ℝ3 with points on S, and prove the following:

Theorem 1. If E is a subset of ℝ3 of Lebesgue measure zero, then for µ almost all directions α, every plane normal to α intersects E in a set of plane measure zero.

Recently the notion of elementary symmetrization attracted new attention in the field of convex sets (see [L; W]), and it was proved that Minkowski's “Quermassintegrale” are decreased by elementary symmetrization. On the other hand, the concept of Schwarz symmetrization for Borel functions gained new interest from its possible applications in the field of elliptic partial differential equations (see [S1, S2, HI, T1, T2, HY1, HY2, GL1, GL2]).

A study complementary to Part 1 (Smith 1979) is made of the linear stability characteristics, at high Reynolds number (R), of Poiseuille How through tubes with closed cross-sections. The first significant deviation of the upper branch of the neutral stability curve (Part 1 having described the lower bilanch) from that of plane Poiseuille flow arises when the aspect ratio is decreased from infinity to O(R1/11). The axial wavenumber α on the upper branch is then O(R-1/11). A further decrease of the aspect ratio, to a finite value, forces this α to fall sharply to O(R-1). A similar phenomenon occurs for the lower branch (Part 1). Thus the two branches are likely to meet only when the aspect ratio becomes finite, with the neutrally stable disturbances then having very large axial length scales.

Let be a finite sequence of positive integers. If we want to show that contains primes, or at least almost-primes (i.e. numbers with few prime factors), sieve methods give better results if weights of a certain kind are attached to the elements of .