International Journal of Mathematics

3-quasi-Sasakian manifolds were studied systematically by the authors in a recent paper as a suitable setting unifying 3-Sasakian and 3-cosymplectic geometries. This paper throws new light on their geometric structure which appears to be generally richer compared to the 3-Sasakian subclass. In fact, it turns out that they are multiply foliated by four distinct fundamental foliations. The study of the transversal geometries with respect to these foliations allows us to link the 3-quasi-Sasakian manifolds to the more famous hyper-Kähler and quaternionic-Kähler geometries. Furthermore, we strongly improve the splitting results previously obtained; we prove that any 3-quasi-Sasakian manifold of rank 4l + 1 is 3-cosymplectic and any 3-quasi-Sasakian manifold of maximal rank is 3-α-Sasakian.

We consider a minimal, free action, φ, of the group ℤ^d on the Cantor set X, for d ≥ 1. We introduce the notion of small positive cocycles for such an action. We show that the existence of such cocycles allows the construction of finite Kakutani–Rohlin approximations to the action. In the case, d = 1, small positive cocycles always exist and the approximations provide the basis for the Bratteli–Vershik model for a minimal homeomorphism of X. Finally, we consider two classes of examples when d = 2 and show that such cocycles exist in both.

In this paper, the authors study the compactness for higher order commutators of oscillatory singular integral operators with rough kernels satisfying L^q-Dini conditions.

We introduce iterated Euler's integrals, and we give expressions using zeta functions. Moreover, we prove that normalized multiple sine functions are expressed via iterated Euler's integrals. Then, we show basic properties of multiple sine functions from these results containing Kummer type formula.

We show that the log canonical bundle, κ, of is very ample, show the homogeneous coordinate ring is Koszul, and give a nice set of rank 4 quadratic generators for the homogeneous ideal: The embedding is equivariant for the symmetric group, and the image lies on many Segre embedded copies of ℙ¹ × ⋯ × ℙ^n-3, permuted by the symmetric group. The homogeneous ideal of is the sum of the homogeneous ideals of these Segre embeddings.