A priori estimate of gradient of a solution of a certain differential inequality and quasiconformal mappings

We prove a global estimate for the gradient of the solution of the Poisson differential inequality |Δu(x)| ≤ a|Du(x)|² + b, x ∈ B ⁿ , where a, b < ∞ and $u|_{S^{n - 1} } \in C^{1,\alpha } (S^{n - 1} ,\mathbb{R}^m )$ . If m = 1 and $a \le (n + 1)/({\left| u \right|_\infty }4n\sqrt n )$ , then |Du| is a priori bounded. This generalizes some similar results due to S. Bernstein [4] and E. Heinz [10] for the plane. An application of these results yields the main result, namely that a quasiconformal mapping of the unit ball onto a domain with C ² smooth boundary satisfying the Poisson differential inequality is Lipschitz continuous. This extends some results of the author, Mateljevi?, and Pavlovi? from the complex plane to ? ⁿ .     

On All Fractional （a,b, k）-Critical Graphs

Let a,b,k,r be nonnegative integers with 1≤a≤b and r≥2.LetG be a graph of order n with n(a+b)(r(a+b)-2)+ak/a.In this paper,we first show a characterization for all fractional(a,b,k)-critical graphs.Then using the result,we prove that G is all fractional(a,b,k)-critical if δ(G)≥(r-1)b2/a+k and |NG(x1)∪NG(x2)∪···∪NG(xr)|≥bn+ak/a+b for any independent subset {x1,x2,...,xr} in G.Furthermore,it is shown that the lower bound on the condition|NG(x1)∪NG(x2)∪···∪NG(xr)|≥bn+ak/a+b is best possible in some sense,and it is an extension of Lu's previous result.     

A generalization of the katona theorem for crosst-intersecting families

LetX be ann-element set and letA and? be families of subsets ofX. We say thatA and? are crosst-intersecting if |A ∩ B| ≥ t holds for all A ∈A and for allB ∈ ?. Suppose thatA and ? are crosst-intersecting. This paper first proves a crosst-intersecting version of Harper's Theorem:

1. There are two crosst-intersecting Hamming spheresA ₀,? ₀ with centerX such that |A| ≤ |A ₀| and|?| ≤ |? ₀| hold.
2. Suppose thatt ≥ 2 and that the pair of integers (|A) is maximal with respect to direct product ordering among pairs of crosst-intersecting families. Then,A and? are Hamming spheres with centerX.

Using these claims, the following conjecture of Frankl is proven:

1. Ifn + t = 2k ? 1 then |A| |?| ≤ max \(\left\{ {\left( {K_k^n + \left( {_{k - 1}^{n - 1} } \right)} \right)^2 ,K_k^n K_{k - 1}^n } \right\}\) holds, whereK _l ⁿ is defined as \(\left( {_n^n } \right)\left( {_{n - 1}^n } \right) + \cdots + \left( {_l^n } \right).\)
2. Ifn + t = 2k then |A| |? ≤ (K _k ⁿ )² holds.

The extremal configurations are also determined.     

Embeddings of weighted sobolev spaces and generalized Caffarelli-Kohn-Nirenberg inequalities

We characterize all the real numbers a, b, c and 1 ?? p, q, r < ?? such that the weighted Sobolev space $$W_{\{ a,b\} }^{\{ q,q\} }({R^N}\backslash \{ 0\} ): = \{ u \in L_{loc}^1({R^N}\backslash \{ 0\} ):{\left| x \right|^{a/q}} \in {L^q}({R^{N),}}{\left| x \right|^{b/p}}\nabla u \in {({L^p}({R^N}))^N}\} $$ is continuously embedded into $${L^r}({R^N};{\left| x \right|^c}dx): = \{ u \in L_{loc}^1({R^N}\backslash \{ 0\} ):{\left| x \right|^{c/r}}u \in {L^r}({R^N})\} $$ with norm ??·?? _c,r . It turns out that, except when N ?? 2 and a = c = b ? p = ?N, such an embedding is equivalent to the multiplicative inequality $${\left\| u \right\|_{c,r}} \le C\left\| {\nabla u} \right\|_{b,p}^\theta \left\| u \right\|_{a,q}^{1 - \theta }$$ for some suitable ?? ?? [0, 1], which is often but not always unique. If a, b, c > ?N, then C ₀ ^?? (? ^N ) ? W _{a,b} ^(q,p) (? ^N {0}) ?? L ^r (? ^N ; |x| ^c dx) and such inequalities for u ?? C ₀ ^?? (? ^N ) are the well-known Caffarelli-Kohn-Nirenberg inequalities; but their generalization to W _{a,b} ^(q,p) (? ^N {0}) cannot be proved by a denseness argument. Without the assumption a, b, c > ?N, the inequalities are essentially new, even when u ?? C ₀ ^?? (? ^N {0}), although a few special cases are known, most notably the Hardy-type inequalities when p = q. In a different direction, the embedding theorem easily yields a generalization when the weights |x| ^a , |x| ^b and |x| ^c are replaced with more general weights w _a ,w _b and w _c , respectively, having multiple power-like singularities at finite distance and at infinity.     

Strongly dependent theories

We further investigate the class of models of a strongly dependent (first order complete) theory T, continuing [Sh:715], [Sh:783] and related works. Those are properties (= classes) somewhat parallel to superstability among stable theory, though are different from it even for stable theories. We show equivalence of some of their definitions, investigate relevant ranks and give some examples, e.g., the first order theory of the p-adics is strongly dependent. The most notable result is: if |A| + |T| ≤ µ, I ? ? and |I|≥?_|T|+(µ), then some J ? I of cardinality µ⁺ is an indiscernible sequence over A.     

Estimates related to sumfree subsets of sets of integers

A subsetA of the positive integers ?₊ is called sumfree provided (A+A)∩A=ø. It is shown that any finite subsetB of ?₊ contains a sumfree subsetA such that |A|≥1/3(|B|+2), which is a slight improvement of earlier results of P. Erdös [Erd] and N. Alon-D. Kleitman [A-K]. The method used is harmonic analysis, refining the original approach of Erdös. In general, defines _k (B) as the maximum size of ak-sumfree subsetA ofB, i.e. (A) _k = $\underbrace {A + ... + A}_{k times}$ % MathType!End!2!1! is disjoint fromA. Elaborating the techniques permits one to prove that, for instance, $s_3 \left( B \right) > \frac{{\left| B \right|}}{4} + c\frac{{\log \left| B \right|}}{{\log \log \left| B \right|}}$ % MathType!End!2!1!as an improvement of the estimate $s_k \left( B \right) > \frac{{\left| B \right|}}{4}$ % MathType!End!2!1! resulting from Erdös’ argument. It is also shown that in an inequalitys _k (B)>δ _k |B|, valid for any finite subsetB of ?₊, necessarilyδ _k → 0 fork → ∞ (which seemed to be an unclear issue). The most interesting part of the paper are the methods we believe and the resulting harmonic analysis questions. They may be worthwhile to pursue.     

Infinitely many small solutions for the p(x)-Laplacian operator with nonlinear boundary conditions

In this paper, we prove the existence of infinitely many small solutions to the following quasilinear elliptic equation ?Δ _p(x) u +  |u| ^p(x)-2 u =  f (x, u) in a smooth bounded domain Ω of ${\mathbb{R}^N}$ with nonlinear boundary conditions ${|\nabla u|^{p-2}\frac{\partial u}{\partial\nu} = |u|^{{q(x)-2}}u}$ . We also assume that ${\{q(x) = p^\ast(x)\}\neq \emptyset}$ , where p*(x) =  Np(x)/(N ? p(x)) is the critical Sobolev exponent for variable exponents. The proof is based on a new version of the symmetric mountain-pass lemma due to Kajikiya, and property of these solutions is also obtained.     

Approximation in commutative Banach algebras with dense principal ideals,II

LetA be a commutative Banach algebra without non-zero idempotents. Then for everya∈A with \(\overline {aA} = A\) there isb∈A be such that every sequence {x _k } inA with \(ax_k \mathop \to \limits^k b\) is unbounded. We give a simple proof of this result.     

Minimum degree of bipartite graphs and the existence ofk-factors

LetG be a bipartite graph with bipartition (X, Y) andk a positive integer. If (i) $$\left| X \right| = \left| Y \right|,$$ (ii) $$\delta (G) \geqslant \left\lceil {\frac{{\left| X \right|}}{2}} \right\rceil \geqslant k,$$ \(\left| X \right| \geqslant 4k - 4\sqrt k + 1\) when |X| is odd and |X| ≥ 4k ? 2 when |X| is even, thenG has ak-factor.     

Integral representations for some classes of functions holomorphic in a strip or in a half-plane

В данной работе рассм атриваются классы фу нкцийf(z), голоморфные в област иa (?∞<a<b≦+∞) приp≧1 иs≧0, и у довлетворяющие одному из следующих условий:

1. Еслиb≦+∞, то $$\int\limits_a^b {(\int\limits_{ - \infty }^{ + \infty } {\left| {f\left( {x + iy} \right)} \right|^p } dy)^s dx< + \infty .} $$
2. Еслиb=+∞, иa=0, то $$\int\limits_0^u {(\int\limits_{ - \infty }^{ + \infty } {\left| {f\left( {x + iy} \right)} \right|^p } dy)^s dx \leqq \varrho \left( u \right), u > 0,} $$ где?(u) — функция опред еленного роста.

Результаты работы су щественно обобщают т еорему Пэли—Винера о параме трическом представлений класс аH ₂ на полуплоскости.     

A smooth global branch of solutions for a semilinear elliptic equation on {\mathbb{R}^N}

The existence of a global branch of positive spherically symmetric solutions ${\{(\lambda,u(\lambda)):\lambda\in(0,\infty)\}}$ of the semilinear elliptic equation $$\Delta u - \lambda u + V(x)|u|^{p-1}u = 0 \quad \text{in}\,\mathbb{R}^N\,\text{with}\,N\geq3$$ is proved for ${1 < p < 1+\frac{4-2b}{N-2}}$ , where ${b\in(0,2)}$ is such that the radial function V vanishes at infinity like |x|^?b . V is allowed to be singular at the origin but not worse than |x|^?b . The mapping ${\lambda\mapsto u(\lambda)}$ is of class ${C^r((0,\infty),H^1(\mathbb{R}^N))}$ if ${V\in C^r(\mathbb{R}^N\setminus\{0\},\mathbb{R})}$ , for r = 0, 1. Further properties of regularity and decay at infinity of solutions are also established. This work is a natural continuation of previous results by Stuart and the author, concerning the existence of a local branch of solutions of the same equation for values of the bifurcation parameter λ in a right neighbourhood of λ = 0. The variational structure of the equation is deeply exploited and the global continuation is obtained via an implicit function theorem.     

A note onL p spaces of harmonic functions

The following result is proved: Letp>0,a>?1. Suppose thatG is a measurable subset ofB, the unit ball in ? ^N , for which there exists a positive constantA ₁, so that $$\int\limits_B {\left( {1 - \left| x \right|} \right)^a \left| {f(x)} \right|^p dm \leqslant A_1 } \int\limits_G {\left( {1 - \left| x \right|} \right)^a \left| {f(x)} \right|^p dm}$$ for each function that is harmonic inB and for which the left-hand side of the above inequality is finite. Then there is a positive constantA ₂ so that for each ballK with center on ?B, $$m\left( {K \cap B} \right) \leqslant A_2 m\left( {K \cap G} \right).$$ Herem denotes Lebesgue measure in ? ^N . This result answers a question left open byDan Luecking [2].     

A Spanning Tree with High Degree Vertices

Let G be a connected graph, let ${X \subset V(G)}$ and let f be a mapping from X to {2, 3, . . .}. Kaneko and Yoshimoto (Inf Process Lett 73:163–165, 2000) conjectured that if |N _G (S) ? X| ≥ f (S) ? 2|S| + ω _G (S) + 1 for any subset ${S \subset X}$ , then there exists a spanning tree T such that d _T (x) ≥ f (x) for all ${x \in X}$ . In this paper, we show a result with a stronger assumption than this conjecture; if |N _G (S) ? X| ≥ f (S) ? 2|S| + α(S) + 1 for any subset ${S \subset X}$ , then there exists a spanning tree T such that d _T (x) ≥ f (x) for all ${x \in X}$ .     

On a certain class of commuting systems of linear operators

In this paper we describe the class of commuting pairs of bounded linear operators {A ₁,A ₂} acting on a Hilbert space H which are unitarily equivalent to the system of integrations over independent variables $$ (\tilde A_1 f)(x,y) = i\int_x^a {f(t,y)} dt, (\tilde A_2 f)(x,y) = i\int_y^b {f(x,s)} ds $$ in $ L_{\Omega _L }^2 $ , where ?? _L is the compact set in ? ₊ ² bounded by the lines x = a and y = b and by a decreasing smooth curve L = {((x, p(x)): p(x) ?? C _[0,a] ¹ , p(0) = b, p(a) = 0}.     

Lower bounds for the modulus of the logarithmic derivative of a polynomial

Estimates are given for the measure of a section of an arbitrary straight line of the set $$E_\delta = \left\{ {z:\left| {P' {{\left( z \right)} \mathord{\left/ {\vphantom {{\left( z \right)} {\left( {nP \left( z \right)} \right)}}} \right. \kern-\nulldelimiterspace} {\left( {nP \left( z \right)} \right)}} \leqslant \delta } \right|} \right\} \left( {\delta > 0} \right)$$ where P (z) is a polynomial of degree n. THEOREM. Suppose P (x) = (x ? x₁) ... (x ? x_n) is a polynomial with real zeros. Then, for any δ > 0, on any intervala ?x ?b, containing all of the x_k (k=1, 2, ..., n), outside an exceptional set E_δ?[a,b] such that $$mes E_\delta \leqslant \left( {\sqrt {1 + \delta ^2 \left( {b - a} \right)^2 } - 1} \right)/\delta $$ , we have the inequality $$\left| {P' {{\left( x \right)} \mathord{\left/ {\vphantom {{\left( x \right)} {\left( {nP \left( x \right)} \right)}}} \right. \kern-\nulldelimiterspace} {\left( {nP \left( x \right)} \right)}}} \right| > \delta $$ . A similar estimate is given for polynomials whose roots lie either in Imz ? 0 or in Imz ? 0.     

Commutators of BMO functions and degenerate Schr?dinger operators with certain nonnegative potentials

Let ${\mathcal{L}f(x)=-\frac{1}{\omega}\sum_{i,j} \partial_i(a_{i,j}(\cdot)\partial_jf)(x)+V(x)f(x)}$ with the non-negative potential V belonging to reverse H?lder class with respect to the measure ??(x)dx, where ??(x) satisfies the A ₂ condition of Muckenhoupt and a _i,j (x) is a real symmetric matrix satisfying ${\lambda^{-1}\omega(x)|\xi|^2\le \sum^n_{i,j=1}a_{i,j}(x)\xi_i\xi_j\le\lambda\omega(x)|\xi|^2. }$ We obtain some estimates for ${V^{\alpha}\mathcal{L}^{-\alpha}}$ on the weighted L ^p spaces and we study the weighted L ^p boundedness of the commutator ${[b, V^{\alpha} \mathcal{L}^{-\alpha}]}$ when ${b\in BMO_\omega}$ and 0?<??? ?? 1.     

Existence and asymptotic behaviour of solutions of the very fast diffusion equation

Let n ≥ 3, 0 < m ≤ (n ? 2)/n, p > max(1, (1 ? m)n/2), and ${0 \le u_0 \in L_{loc}^p(\mathbb{R}^n)}$ satisfy ${{\rm lim \, inf}_{R\to\infty}R^{-n+\frac{2}{1-m}} \int_{|x|\le R}u_0\,dx = \infty}$ . We prove the existence of unique global classical solution of u _t = Δu ^m , u > 0, in ${\mathbb{R}^n \times (0, \infty), u(x, 0) = u_0(x)}$ in ${\mathbb{R}^n}$ . If in addition 0 < m < (n ? 2)/n and u ₀(x) ≈ A|x|^?q as |x| → ∞ for some constants A > 0, q < n/p, we prove that there exist constants α, β, such that the function v(x, t) = t ^α u(t ^β x, t) converges uniformly on every compact subset of ${\mathbb{R}^n}$ to the self-similar solution ψ(x, 1) of the equation with ψ(x, 0) = A|x|^?q as t → ∞. Note that when m = (n ? 2)/(n + 2), n ≥ 3, if ${g_{ij} = u^{\frac{4}{n+2}}\delta_{ij}}$ is a metric on ${\mathbb{R}^n}$ that evolves by the Yamabe flow ?g _ij /?t = ?Rg _ij with u(x, 0) = u ₀(x) in ${\mathbb{R}^n}$ where R is the scalar curvature, then u(x, t) is a global solution of the above fast diffusion equation.     

Boundary parametrization of planar self-affine tiles with collinear digit set

We consider a class of planar self-affine tiles T = M-1 a∈D(T + a) generated by an expanding integral matrix M and a collinear digit set D as follows:M =(0-B 1-A),D = {(00),...,(|B|0-1)}.We give a parametrization S1 →T of the boundary of T with the following standard properties.It is H¨older continuous and associated with a sequence of simple closed polygonal approximations whose vertices lie on T and have algebraic preimages.We derive a new proof that T is homeomorphic to a disk if and only if 2|A| |B + 2|.     

Liouville Type Theorems for Elliptic Equations with Gradient Terms

In this paper we obtain Liouville type theorems for nonnegative supersolutions of the elliptic problem ${-\Delta u + b(x)|\nabla u| = c(x)u}$ in exterior domains of ${\mathbb{R}^N}$ . We show that if lim ${{\rm inf}_{x \longrightarrow \infty} 4c(x) - b(x)^2 > 0}$ then no positive supersolutions can exist, provided the coefficients b and c verify a further restriction related to the fundamental solutions of the homogeneous problem. The weights b and c are allowed to be unbounded. As an application, we also consider supersolutions to the problems ${-\Delta u + b|x|^{\lambda}|{\nabla} u| = c|x|^{\mu} u^p}$ and ${-\Delta u + be^{\lambda |x|}|\nabla u| = ce^{\mu |x|}u^p}$ , where p > 0 and λ, μ ≥ 0, and obtain nonexistence results which are shown to be optimal.