9,060 research outputs found
Posets arising as 1-skeleta of simple polytopes, the nonrevisiting path conjecture, and poset topology
Given any polytope and any generic linear functional , one
obtains a directed graph by taking the 1-skeleton of and
orienting each edge from to for .
This paper raises the question of finding sufficient conditions on a polytope
and generic cost vector so that the graph will
not have any directed paths which revisit any face of after departing from
that face. This is in a sense equivalent to the question of finding conditions
on and under which the simplex method for linear programming
will be efficient under all choices of pivot rules. Conditions on and are given which provably yield a corollary of the desired face
nonrevisiting property and which are conjectured to give the desired property
itself. This conjecture is proven for 3-polytopes and for spindles having the
two distinguished vertices as source and sink; this shows that known
counterexamples to the Hirsch Conjecture will not provide counterexamples to
this conjecture.
A part of the proposed set of conditions is that be the
Hasse diagram of a partially ordered set, which is equivalent to requiring non
revisiting of 1-dimensional faces. This opens the door to the usage of
poset-theoretic techniques. This work also leads to a result for simple
polytopes in which is the Hasse diagram of a lattice L that the
order complex of each open interval in L is homotopy equivalent to a ball or a
sphere of some dimension. Applications are given to the weak Bruhat order, the
Tamari lattice, and more generally to the Cambrian lattices, using realizations
of the Hasse diagrams of these posets as 1-skeleta of permutahedra,
associahedra, and generalized associahedra.Comment: new results for 3-polytopes and spindles added; exposition
substantially improved throughou
Professional Ethics and Social Responsibility: Military Work and Peacebuilding
The paper considers a number of important questions related to the involvement of engineers in peacebuilding and military work, including the preference of many countries for high tech weapons based security over peacebuilding, whether and in what circumstances, if any, it is justified for engineers to be involved in military work; and how engineers can persuade their colleagues to apply their skills to support peacebuilding. It is introduced by an overview of what is meant by the term military work and the extent and consequences of the use of military technology worldwide. This is followed by the applications of different approaches and theories of ethics to discuss the questions presented in the introduction. The approaches and theories applied include considerations of micro-and macro-ethics, codes of ethics, virtue ethics, considerations of gender and paradigms and the ethical imperative. Initial insights include the importance of considering the associated context and the need to avoid othering, which can make different treatment of minority groups, including the use of high tech weapons against them, seem acceptable
Shelling Coxeter-like Complexes and Sorting on Trees
In their work on `Coxeter-like complexes', Babson and Reiner introduced a
simplicial complex associated to each tree on nodes,
generalizing chessboard complexes and type A Coxeter complexes. They
conjectured that is -connected when the tree has
leaves. We provide a shelling for the -skeleton of , thereby
proving this conjecture.
In the process, we introduce notions of weak order and inversion functions on
the labellings of a tree which imply shellability of , and we
construct such inversion functions for a large enough class of trees to deduce
the aforementioned conjecture and also recover the shellability of chessboard
complexes with . We also prove that the existence or
nonexistence of an inversion function for a fixed tree governs which networks
with a tree structure admit greedy sorting algorithms by inversion elimination
and provide an inversion function for trees where each vertex has capacity at
least its degree minus one.Comment: 23 page
Regular cell complexes in total positivity
This paper proves a conjecture of Fomin and Shapiro that their combinatorial
model for any Bruhat interval is a regular CW complex which is homeomorphic to
a ball. The model consists of a stratified space which may be regarded as the
link of an open cell intersected with a larger closed cell, all within the
totally nonnegative part of the unipotent radical of an algebraic group. A
parametrization due to Lusztig turns out to have all the requisite features to
provide the attaching maps. A key ingredient is a new, readily verifiable
criterion for which finite CW complexes are regular involving an interplay of
topology with combinatorics.Comment: accepted to Inventiones Mathematicae; 60 pages; substantially revised
from earlier version
Overcoming barriers and increasing independence: service robots for elderly and disabled people
This paper discusses the potential for service robots to overcome barriers and increase independence of
elderly and disabled people. It includes a brief overview of the existing uses of service robots by disabled and elderly
people and advances in technology which will make new uses possible and provides suggestions for some of these new
applications. The paper also considers the design and other conditions to be met for user acceptance. It also discusses
the complementarity of assistive service robots and personal assistance and considers the types of applications and
users for which service robots are and are not suitable
Ethical Engineering and Respect for The ‘Other’
Engineers have a very important role and responsibility in shaping modern society. Diversity amongst engineers is important in fulfilling this responsibility and ensuring that the creativity and needs of the whole population are taken account of. However, only a small percentage of engineers are female and very few of them are disabled. The paper discusses the experiences of women and disabled engineers in the context of othering and considers the way in which the existence of binary divides facilitates marginalisation and exclusion. It also discusses the need to involve end-users in design and development and education to encourage this, with a particular focus on disabled end-users
Fatigue of boron-aluminum composites bonds and joints
Study examines effects of boron filament diameter on bonds and joints in boron-aluminum composite. Data include static strength, fatigue, and dynamic moduli of elasticity. Manson-Coffin analyses and metallurgical and fracture surface evaluation were also performed
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