On geodesic envelopes and caustics

We give a global description of envelopes of geodesic tangents of regular curves in (not necessarily convex) Riemannian surfaces. We prove that such an envelope is the union of the curve itself, its inflectional geodesics and its tangential caustics (formed by the conjugate points to those of the initial curve along the tangent geodesics). Stable singularities of tangential caustics and geodesic envelopes are discussed. We also prove the (global) stability of tangential caustics of close curves in convex closed surfaces under small deformations of the initial curve and of the ambient metric.Comment: 7 pp. 1 figure. 2nd versio

Blocks of the category of smooth ℓ\ell-modular representations of GL(n,F)GL(n,F) and its inner forms: reduction to level-00

Let $G$ be an inner form of a general linear group over a non-archimedean locally compact field of residue characteristic $p$, let $R$ be an algebraically closed field of characteristic different from $p$ and let $\mathscr{R}_R(G)$ be the category of smooth representations of $G$ over $R$. In this paper, we prove that a block (indecomposable summand) of $\mathscr{R}_R(G)$ is equivalent to a level-$0$ block (a block in which every object has non-zero invariant vectors for the pro-$p$-radical of a maximal compact open subgroup) of $\mathscr{R}_R(G')$, where $G'$ is a direct product of groups of the same type of $G$

Simple tangential families and perestroikas of their envelopes

Tangential families are 1-parameter families of rays emanating tangentially from smooth curves. We classify tangential family germs up to Left-Right equivalence: we prove that there are two infinite series and four sporadic simple singularities of tangential family germs (in addition to two stable singularities). We give their normal forms and miniversal tangential deformations (i.e., deformations among tangential families), and we describe the corresponding envelope perestroikas of small codimension. We also discuss envelope singularities of non simple tangential families.Comment: 11 page

Legendrian graphs generated by Tangential Families

We construct a Legendrian version of Envelope theory. A tangential family is a 1-parameter family of rays emanating tangentially from a smooth plane curve. The Legendrian graph of the family is the union of the Legendrian lifts of the family curves in the projectivized cotangent bundle $PT^*\R^2$. We study the singularities of Legendrian graphs and their stability under small tangential deformations. We also find normal forms of their projections into the plane. This allows to interprete the beaks perestroika as the apparent contour of a deformation of the Double Whitney Umbrella singularity $A_1^\pm$

Generic singularities of minimax solutions to Hamilton--Jacobi equations

Minimax solutions are weak solutions to Cauchy problems involving Hamilton--Jacobi equations, constructed from generating families quadratic at infinity of their geometric solutions. We give a complete description of minimax solutions and we classify their generic singularities of codimension not greater than 2.Comment: To appear in Journal of Geometry and Physic

Stable tangential families and singularities of their envelopes

We study tangential families, i.e. systems of rays emanating tangentially from given curves. We classify, up to Left-Right equivalence, stable singularities of tangential family germs (under deformations among tangential families) and we study their envelopes. We discuss applications of our results to the case of tangent geodesics of a curve

The birth of roboethics

The importance, and urgency, of a Roboethics lay in the lesson of our recent history. Two of the front rank fields of science and technology, Nuclear Physics and Genetic Engineering, have already been forced to face the ethical consequences of their research’s applications under the pressure of dramatic and troubling events. In many countries, public opinion, shocked by some of these effects, urged to either halt the whole applications, or to seriously control them. Robotics is rapidly becoming one of the leading field of science and technology, so that we can forecast that in the XXI century humanity will coexist with the first alien intelligence we have ever come in contact with - robots. It will be an event rich in ethical, social and economic problems. Public opinion is already asking questions such as: “Could a robot do "good" and "evil”? “Could robots be dangerous for humankind?”

Thresholds for hanger slackening and cable shortening in the Melan equation for suspension bridges

The Melan equation for suspension bridges is derived by assuming small displacements of the deck and inextensible hangers. We determine the thresholds for the validity of the Melan equation when the hangers slacken, thereby violating the inextensibility assumption. To this end, we preliminarily study the possible shortening of the cables: it turns out that there is a striking difference between even and odd vibrating modes since the former never shorten the cables. These problems are studied both on beams and plates