Let be a covering projection. We use to denote the map defined by , where spaces of continuous maps are endowed with the compact-open topology. We prove that if is Hausdorff and contractible, or if is either a compact CW complex or a graph with finitely many components and is onto, then is a...

Recall that the is a non-commutative algebra over a field k with two generators x and y satisfying Lie relation [x,y]=xy−yx = 1. In 1968 Jacques Dixmier stated a conjecture as follows: Every non zero homomorphism from A(k)into itself is an isomorphism. Until now, it is not known that Dixmier's...

In this note, we attack a question posed ten years ago by Tsukiyama about the injectivity of the so-called Forgetful map. We show that we can insert the Forgetful map in an exact sequence and that the problem can be reduced to the computation of the sequence which turns out unexpectedly to be...

In this paper we study packing the sequence of gracefully labelled trees , , …, with 1, 2, .., − 1 edges into the complete graph . It is shown that if ∈ {, } where and are, respectively, a star tree and a path on vertices then the packing is possible

Here we consider time-dependent linear systems of the formwith state ∈ , control (input) ∈ , and output ∈ . The main results of this paper are local characterizations of observability and strong observability (or observability with unknown inputs) of (, ) and (, , ). These criteria are...

It is shown in this note that a connected block design is a -design if and only if its dual design is a -design. The result was known for proper and equi-replicate designs [e.g., Saha (1976)]. An immediate application of this property is that we can always give a -design with , to save...

The first part of this paper gives the complete equivariant classification of smooth actions on closed, orientable, 8-dimensional manifolds in terms of the weighted quotient space and a lifting obstruction, when the principal isotropy is the identity. The second part addresses the question of a...

Bochner's technique is applied to the study of timelike vectors fields on a Lorentzian manifold. In the compact case, a Lorentzian Bochner integral formula is obtained. As a consequence, compact Ricci flat Lorentzian manifolds admitting a timelike conformal vector field are classified. Both in...

We have given an forbidden subgraph (web-trap) which is an extention of Thomassen's characterization of the rectangular drawing of plane graphs with maximum vertex degree four. A drawing algorithm is also proposed