The logarithm of the modulus of a holomorphic function as a minorant for a subharmonic function

For an arbitrary subharmonic function not identically equal to ?∞ in a domain D of the complex plane C, we prove the existence of a nonzero holomorphic function in D the logarithm of whose modulus is majorized by locally averaging a subharmonic function with logarithmic additions or even without them in the case D = C.     

A note on the existence of non-cyclic free subgroups in division rings

A division ring D is said to be weakly locally finite if for every finite subset ${S \subset D}$ , the division subring of D generated by S is centrally finite. It is known that the class of weakly locally finite division rings strictly contains the class of locally finite division rings. In this note we prove that every non-central subnormal subgroup of the multiplicative group of a weakly locally finite division ring contains a non-cyclic free subgroup. This generalizes the previous result by Gonçalves for centrally finite division rings.     

Circular law, extreme singular values and potential theory

Consider the empirical spectral distribution of complex random n×n matrix whose entries are independent and identically distributed random variables with mean zero and variance 1/n. In this paper, via applying potential theory in the complex plane and analyzing extreme singular values, we prove that this distribution converges, with probability one, to the uniform distribution over the unit disk in the complex plane, i.e. the well known circular law, under the finite fourth moment assumption on matrix elements.     

On the supersoluble residual of a product of subnormal supersoluble subgroups

We give a sufficient condition for supersolubility of a finite group that is a product of two subnormal supersoluble subgroups. We prove that the supersoluble residual of such a group is equal to its nilpotent residual. Also we apply these results to finite groups that are a product of two subnormal p-supersoluble subgroups.     

Order of approximation by electrostatic fields due to electrons

LetD be a Jordan domain in the complex plane andA _q (D) the Bers space with norm ∥ ∥ _q . IfD is the unit disk, it is known that ∥S _n 0∥₂≥π/18, whereS _n =∑ _k=1 ⁿ l/(z?z _nk ) withz _nk ∈?D, so that approximation in ∥ ∥ _q ,q<-2, is not possible. In this paper, we give an order of estimate of ∥f?S _n ∥ _q for 2<q<∞ when ?D is a sufficiently smooth Jordan curve, and prove that this order of approximation is in general best possible.     

On Doob's conjecture about fine limit and Julia point

In the 1960, professor Doob proved that if f(z) is a meromorphic function on a disk D, then at almost every point of the perimeter of D either (a) f has both an angular limit and an equal fine limit, (b) f has every point of the plane as cluster value in every angle opening into D with vertex at the point, and f has a fine limit at the point, or (c) f has every point of the plane both as cluster value in every angle opening into D with vertex at the point and as fine cluster value. He then mentioned that it would be interesting to find an example to show that case (b) can really occur on a perimeter set of strictly positive measure. In this article, we prove this conjecture to be true.     

Toric sets and orbits on toric varieties

Let D be an integer matrix. A toric set, namely the points in Kⁿ parametrized by the columns of D, and a toric variety are associated to D. The toric set is a subset of the toric variety. We describe the relation between the toric set and the toric variety, in terms of the orbits of the torus action on the toric variety. The toric set depends on the sign (+,−,0) pattern of the matrix D. Finally, we prove that any toric variety over an algebraically closed field can be expressed as a toric set, for an appropriate matrix.     

On operator-valued analytic functions with constant norm

Let X be a complex Banach space and $\text{D}$ a domain in the complex plane. Let f: $\text{D}$ → X be an analytic function such that ∥f(ζ)∥ is constant as ζ ? $\text{D}$. If X is the complex plane, then by the classical maximum modulus theorem f;(ζ) itself is constant on $\text{D}$. This is not the case in general. In the paper we study the norm-constant analytic functions whose values are bounded linear operators over an uniformly convex complex Banach space or, in particular, over a complex Hilbert space.     

The Hessenberg matrix and the Riemann mapping function

We consider a Jordan arc Γ in the complex plane ${\mathbb C}$ and a regular measure μ whose support is Γ. We denote by D the upper Hessenberg matrix of the multiplication by z operator with respect to the orthonormal polynomial basis associated with μ. We show in this work that, if the Hessenberg matrix D is uniformly asymptotically Toeplitz, then the symbol of the limit operator is the restriction to the unit circle of the Riemann mapping function ?(z) which maps conformally the exterior of the unit disk onto the exterior of the support of the measure μ. We use this result to show how to approximate the Riemann mapping function for the support of μ from the entries of the Hessenberg matrix D.     

On the structure of the adjacency matrix of the line digraph of a regular digraph

We show that the adjacency matrix M of the line digraph of a d-regular digraph D on n vertices can be written as M=AB, where the matrix A is the Kronecker product of the all-ones matrix of dimension d with the identity matrix of dimension n and the matrix B is the direct sum of the adjacency matrices of the factors in a dicycle factorization of D.     

Extension of two theorems of Payne to some nonlinear Dirichlet problems

In this paper we prove that under certain convexity and symmetry assumptions on a domain in the plane any positive solutionu, of Δu+λf(u)=0, in,D,u=0 on ?D has only one interior critical point. This extends results of L. E. Payne [1].     

On the structure of 3-nets embedded in a projective plane

We investigate finite 3-nets embedded in a projective plane over a (finite or infinite) field of any characteristic p. Such an embedding is regular when each of the three classes of the 3-net comprises concurrent lines, and irregular otherwise. It is completely irregular when no class of the 3-net consists of concurrent lines. We are interested in embeddings of 3-nets which are irregular but the lines of one class are concurrent. For an irregular embedding of a 3-net of order n?5 we prove that, if all lines from two classes are tangent to the same irreducible conic, then all lines from the third class are concurrent. We also prove the converse provided that the order n of the 3-net is smaller than p. In the complex plane, apart from a sporadic example of order n=5 due to Stipins [7], each known irregularly embedded 3-net has the property that all its lines are tangent to a plane cubic curve. Actually, the procedure of constructing irregular 3-nets with this property works over any field. In positive characteristic, we present some more examples for n?5 and give a complete classification for n=4.     

Stability and D-stability

In this paper, we prove a new sufficient condition for D-stability of a matrix. We then use it to show that (i) a nonderogatory stable matrix A has a Schwarz canonical form which is D-stable, and (ii) a nonderogatory stable matrix A has a Routh-canonical form which is totally stable.     

Contribution to the General Linear Conjugation Problem for A Piecewise Analytic Vector

Establishing an analogy between the theories of Riemann–Hilbert vector problem and linear ODEs, for the n-dimensional homogeneous linear conjugation problem on a simple smooth closed contour Γ partitioning the complex plane into two domains D⁺ and D^? we show that if we know n?1 particular solutions such that the determinant of the size n?1 matrix of their components omitting those with index k is nonvanishing on D⁺ ∪ Γ and the determinant of the matrix of their components omitting those with index j is nonvanishing on Γ ∪ D^? {∞}, where \(k,j = \overline {1,n} \), then the canonical system of solutions to the linear conjugation problem can be constructed in closed form.     

Real hypersurfaces in a complex space form with a condition on the structure Jacobi operator

In this paper we classify the real hypersurfaces in a non-flat complex space form with its structure Jacobi operator R _ξ satisfying (? _X R _ξ )ξ = 0, for all vector fields X in the maximal holomorphic distribution D. With this result, we prove the non-existence of real hypersurfaces with D-parallel as well as D-recurrent structure Jacobi operator in complex projective and hyperbolic spaces. We can also prove the non-existence of real hypersurfaces with recurrent structure Jacobi operator in a non-flat complex space form as a corollary.     

A sufficient connectivity condition for generic rigidity in the plane

A graph G=(V,E) is said to be 6-mixed-connected if G−U−D is connected for all sets U⊆V and D⊆E which satisfy 2|U|+|D|≤5. In this note we prove that 6-mixed-connected graphs are (redundantly globally) rigid in the plane. This improves on a previous result of Lovász and Yemini.     

Cochain algebra on manifolds and convergence under refinement

In this paper we develop several algebraic structures on the simplicial cochains of a triangulated manifold and prove they converge to their differential-geometric analogues as the triangulation becomes small. The first such result is for a cochain cup product converging to the wedge product on differential forms. Moreover, we show any extension of this product to a C_∞-algebra also converges to the wedge product of forms. For cochains equipped with an inner product, we define a combinatorial star operator and show that for a certain cochain inner product this operator converges to the smooth Hodge star operator.     

Desarguesian nets without ovals

Let Π be the Desarguesian affine plane coordinatized by a division ring D with charD≠2. For n=6 and also for n=7, we prove that Π holds an n-net without an oval if and only if |D|?9.     

On q-orthogonal polynomials over a collection of complex origin intervals related to little q-Jacobi polynomials

We construct the sequence of orthogonal polynomials with respect to an inner product which is defined by q-integrals over a collection of intervals in the complex plane. We prove that they are connected with little q-Jacobi polynomials. For such polynomials we discuss a few representations, a recurrence relation, a difference equation, a Rodrigues-type formula and a generating function. 2000 Mathematics Subject Classification Primary—33D45, 42C05