A short proof of the nonuniform Ray-Chaudhuri-Wilson inequality

LetL be a set ofs nonnegative integers and a family of subsets of ann-element setX. Suppose that for any two distinct membersA,B we have¦A B¦ L. Assuming in addition that, is uniform, i.e. each member of has the same cardinality, a celebrated theorem of D. K. Ray-Chaudhuri and R. M. Wilson asserts that ¦¦ P. Frankl and R. M. Wilson proved that without the uniformity assumption, we have.We give a short proof of this latter result.     

A topological obstruction to the geodesibility of a foliation of odd dimension

Let M be a compact Riemannian manifold of dimension n, and let be a smooth foliation on M. A topological obstruction is obtained, similar to results of R. Bott and J. Pasternack, to the existence of a metric on M for which is totally geodesic. In this case, necessarily that portion of the Pontryagin algebra of the subbundle must vanish in degree n if is odd-dimensional. Using the same methods simple proofs of the theorems of Bott and Pasternack are given.     

Decomposition of large uniform hypergraphs

We determine a minimum cardinality family _{n, k} (resp. _{n, k} ) ofn-uniform,k-edge hypergraphs satisfying the following property: all, except for finitely many,n-uniform hypergraphs satisfying the divisibility condition have an _{n, k} -decomposition (resp. vertex _{n, k} -decomposition).     

Strong Laws for Martingale Differences and Independent Random Variables

A strong law of large numbers (SLLN) for martingale differences {X _n,_n,n1} permitting constant, random or hybrid normalizations, is obtained via a related SLLN for their conditional variances E{X _n ² |_n-1}n1. This, in turn, leads to martingale generalizations of known results for sums of independent random variables. Moreover, in the independent case, simple conditions are given for a generalized SLLN which contains the classical result of Kolmogorov when the variables are i.i.d.     

On spaces of Baire I functions over K-analytic spaces

Suppose that is a relatively countably compact subset of B₁(X), the space of Baire I functions over a K-analytic space X equipped with the pointwise convergence topology. It is proved that (1) the closure of is a strongly countably compact Frechét-Urysohn space; (2) if is ₁ -compact, is a bicompactum; (3) if X is a paracompact space, the closure of is a bicompactum.Translated from Matematicheskie Zametki, Vol. 52, No. 3, pp. 108–116, September, 1992.     

Ramsey families which exclude a graph

For graphsA andB the relationA(B) _r ¹ means that for everyr-coloring of the vertices ofA there is a monochromatic copy ofB inA. Forb (G) is the family of graphs which do not embedG. A familyof graphs is Ramsey if for all graphsBthere is a graphAsuch thatA(B) _r ¹ . The only graphsG for which it is not known whether Forb (G) is Ramsey are graphs which have a cutpoint adjacent to every other vertex except one. In this paper we prove for a large subclass of those graphsG, that Forb (G) does not have the Ramsey property.This research has been supported in part by NSERC grant 69-1325.     

Happy families and completely Ramsey sets

Summary We use games of Kastanas to obtain a new characterization of the classC of all sets that are completely Ramsey with respect to a given happy family . We then combine this with ideas of Plewik to give a uniform proof of various results of Ellentuck, Louveau, Mathias and Milliken concerning the extent ofC . We also study some cardinals that can be associated with the ideal of nowhere -Ramsey sets.Part of this research was done while the author was visiting I.V.I.C. in Caracas in September 1989. The author would like to thank Carlos Di Prisco for his hospitality.     

Flowchart machines

Let denote a conventional flowchart. Any algorithm can be represented by a flowchart. If action nodes in call then is a recursive flowchart. We show how to decompose arbitrary non-self-modifying programs into structure and atomic parts. We specifically give the synthesis procedure for a controller . can serve as the only sequencer in an execution of . If is recursive then is a pushdown machine, otherwise is a finite state machine. The next-state functionf and the output functiong of represent respectively all of the structure-, i.e. the programmer-oriented-, and all of the atomic-, i.e. the data-oriented-, parts of .f defines the flow or pattern of computations andg the actual transformations or operations on data. Thus we construct and analyze programs by constructing and analyzing their sequencers .     

Generation of Uniformly Closed Algebras of Functions

For a linear sublattice of C(X), the set of all real continuous functions on the completely regular space X, we denote by A() the smallest uniformly closed and inverse-closed subalgebra of C(X) that contains . In this paper we study different methods to generate A() from . For that, we introduce some families of functions which are defined in terms of suprema or sums of certain countably many functions in . And we prove that A() is the uniform closure of each of these families. We obtain, in particular, a generalization of a known result about the generation of A() when is a uniformly closed linear sublattice of bounded functions.     

Set systems with no union of cardinality 0 modulom

Letq be a prime power. It is shown that for any hypergraph = {F ₁,...,F _d(q–1)+1} whose maximal degree isd, there exists Ø ₀ , such that 0 (modq).     

Convexly independent sets

A family of pairwise disjoint compact convex sets is called convexly independent, if none of its members is contained in the convex hull of the union of the other members of the family. The main result of the paper gives an upper bound for the maximum cardinalityh(k, n) of a family of mutually disjoint compact convex sets such that any subfamily of at mostk members of is convexly independent, but no subfamily of sizen is.     

Conical flocks,partial flocks,derivation, and generalized quadrangles

L. Bader, G. Lunardon and J. A. Thas have shown that a flock ₀ of a quadratic cone in PG(3, q), q odd, determines a set ={⁰,₁,...,_q} of q+1 flocks. Each _j , 1jq, is said to be derived from ₀. We show that, by derivation, the flocks with q=3 ^e arising from the Ganley planes yield an inequivalent flock for q27. Further, we prove that the Fisher flocks (q odd, q5) are the unique nonlinear flocks for which (q–1)/2 planes of the flock contain a common line. This result is used to show that each of the flocks derived from a Fisher flock is again a Fisher flock. Finally, we prove that any set of q–1 pairwise disjoint nonsingular conics of a cone can be extended to a flock. All these results have implications for the theory of translation planes.     

Generalized sentinels defined via least squares

We address the problem of monitoring a linear functional (c, x)_Eof an unknown vectorx of a Hilbert spaceE, the available data being the observationz, in a Hilbert spaceF, of a vectorAx depending linearly onx through some known operatorA(E; F). WhenE=E ₁×E ₂,c=(c ₁ 0), andA is injective and defined through the solution of a partial differential equation, Lions ([6]–[8]) introduced sentinelssF such that (s, Ax)_Fis sensitive to x₁ E ₁ but insensitive to x₂ E₂. In this paper we prove the existence, in the general case, of (i) a generalized sentinel (s, ) ×E, where F withF dense in 80, such that for anya priori guess x₀ ofx, we have s, Ax + (, x₀)_E=(c, x)_E, where x is the least-squares estimate ofx closest tox ₀, and (ii) a family of regularized sentinels (s _n , _n ) F×E which converge to (s, ). Generalized sentinels unify the least-squares approach (by construction !) and the sentinel approach (whenA is injective), and provide a general framework for the construction of sentinels with special sensitivity in the sense of Lions [8]).     

n-Cohomology of simple highest weight modules on walls and purity

Summary LetGPB be respectively a complex connected linear algebraic semisimple group, a parabolic subgroup and a Borel subgroup. The first main result is the following theorem: Let be a pure complex onG/B, smooth with respect to Bruhat cells. Then its restriction to anyP-orbit is pure as well, of the same weight. As a consequence we are able to compute then-cohomology of simple highest weight modules on walls.Written during the author's stay at MSRI, supported by a Stipendium der Clemens-Plassmann-Stiftung     

Superficie cubiche di\mathbb{A}_\mathbb{C}^3 a curve sottoinsieme intersezione completa

Summary In this paper we classify the algebraic cubic surfaces of the affine space is the complex field, whose algebraic curves are set-theoretic complete intersections of ; in other words surfaces such that every prime ideal of height 1in the coordinate ring [] of is the radical of a principal ideal; if is non singular in codimension 1this means that [] is semifactorial. We give the equations of such surfaces within linear isomorphisms of providing also methods by which one can construct the equations of the surfaces cutting on its curves as set-theoretic complete intersections. Moreover for each of these surfaces we determine the minimum positive number such that every algebraic curve of with multiplicity of intersection , is complete intersection of itself with another surface § 8where the results are summarized). We tackle also the problem of such a classification over algebraically closed fields k different from .

---

---

Lavoro eseguito nell'ambito del G.N.S.A.G.A. del C.N.R.     

Smoothing the function of the multiple-shooting equation

The multiple-shooting method for the solution of boundary-value problems is a modified Newton method for the solution of an equation(x) = 0, where is a special function which is differentiable in general, but may occasionally have discontinuities at some points which have to be passed during the iteration process. This is the case especially in optimal control problems and it is a severe handicap for the convergence of the Newton method which can be essentially reduced when replacing by a series of smooth functions _i dependent on the iteration process.     

Rate of convergence in the central limit theorem for empirical processes

Let _n and be an empirical process and a generalized Brownian bridge, respectively, indexed by a class of real measurable functions. From the central limit theorem for empirical processes it follows that for allr0. In this paper, assuming the class to be countably determined, under certain conditions we obtain an estimate for some constantC. Vapnik-ervonenkis class and the indicators of lower left orthants provide examples of classes considered here.     

A sharp partitioning-inequality for non-atomic probability measures based on the mass of the infimum of the measures

Summary If ₁, ... , are non-atomic probability measures on the same measurable space (S, ), then there is an -measurable partition {A _i } _{i = 1} ⁿ of S so that _i (A _i )(n – 1 + m)^–1 for all i=1, ..., n, where is the total mass of the largest measure dominated by each of the _i S; moreover, this bound is attained for all n1 and all m in [0, 1]. This result is an analog of the bound (n+1-M) ^-1of Elton et al. [5] based on the mass M of the supremum of the measures; each gives a quantative generalization of a well-known cake-cutting inequality of Urbanik [10] and of Dubins and Spanier [2].Research partly supported by NSF Grants DMS-84-01604 and DMS-86-01608     

Facets and lifting procedures for the set covering polytope

Given a family of subsets of an arbitrary groundsetE, acover of is any setC E having non-empty intersection with every subset in. In this paper we deal with thecovering polytope, i.e., the convex hull of the incidence vectors of all the covers of. In Section 2 we review all the known properties of the covering polytope. In Sections 3 and 4 we introduce two new classes of non-Boolean facets of such a polytope. In Sections 5 and 6 we describe some non-sequential lifting procedures. In Section 7 a generalization of the notion ofweb introduced by L.E. Trotter is presented together with the facets of the covering polytope produced by such a structure.Moreover, the strong connections between several combinatorial problems and the covering problem are pointed out and, exploiting those connections, some examples are presented of new facets for the Knapsack and Acyclic Subdigraph polytopes.     

Blow-up rates for parabolic systems

Let ⁿ be a bounded domain andB _R be a ball in ⁿ of radiusR. We consider two parabolic systems: ut=u +f(), i= +g(u) in × (0,T) withu=v=0 on × (0,T) andu _t =u, v _t =v inB _r × (0,T) withe/v=f (v), e/v=g(u) onB _R × (0,T). Whenf(v) andg(u) are power law or exponential functions, we establish estimates on the blow-up rates for nonnegative solutions of the systems.