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1.
Let A and B denote two families of subsets of an n-element set. The pair (A,B) is said to be ℓ-cross-intersecting iff |A∩B|=ℓ for all A∈ A and B∈B. Denote by P
e
(n) the maximum value of |A||B| over all such pairs. The best known upper bound on P
e
(n) is Θ(2
n
), by Frankl and R?dl. For a lower bound, Ahlswede, Cai and Zhang showed, for all n ≥ 2ℓ, a simple construction of an ℓ-cross-intersecting pair (A,B) with |A||B| = $
\left( {{*{20}c}
{2\ell } \\
\ell \\
} \right)
$
\left( {\begin{array}{*{20}c}
{2\ell } \\
\ell \\
\end{array} } \right)
2
n−2ℓ
= Θ(2
n
/$
\sqrt \ell
$
\sqrt \ell
), and conjectured that this is best possible. Consequently, Sgall asked whether or not P
e
(n) decreases with ℓ. 相似文献
2.
Assume that no cardinal κ < 2
ω
is quasi-measurable (κ is quasi-measurable if there exists a κ-additive ideal
$
\mathbb{I}
$
\mathbb{I}
of X contains uncountably many pairwise disjoint subfamilies
$
\mathbb{I}
$
\mathbb{I}
-Bernstein unions ∪
$
\mathbb{I}
$
\mathbb{I}
-Bernstein if A and X \ A meet each Borel $
\mathbb{I}
$
\mathbb{I}
-positive subset B ⊆ X). This result is a generalization of the Four Poles Theorem (see [1]) and results from [2] and [4]. 相似文献
3.
We obtain characterizations (and prove the corresponding equivalence of norms) of function spaces B
pq
sm
($
\mathbb{I}
$
\mathbb{I}
k
) and L
pq
sm
($
\mathbb{I}
$
\mathbb{I}
k
) of Nikol’skii-Besov and Lizorkin-Triebel types, respectively, in terms of representations of functions in these spaces by
Fourier series with respect to a multiple system $
\mathcal{W}_m^\mathbb{I}
$
\mathcal{W}_m^\mathbb{I}
of Meyer wavelets and in terms of sequences of the Fourier coefficients with respect to this system. We establish order-sharp
estimates for the approximation of functions in B
pq
sm
($
\mathbb{I}
$
\mathbb{I}
) and L
pq
sm
($
\mathbb{I}
$
\mathbb{I}
k
) by special partial sums of these series in the metric of L
r
($
\mathbb{I}
$
\mathbb{I}
k
) for a number of relations between the parameters s, p, q, r, and m (s = (s
1, ..., s
n
) ∈ ℝ+
n
, 1 ≤ p, q, r ≤ ∞, m = (m
1, ..., m
n
) ∈ ℕ
n
, k = m
1 +... + m
n
, and $
\mathbb{I}
$
\mathbb{I}
= ℝ or $
\mathbb{T}
$
\mathbb{T}
). In the periodic case, we study the Fourier widths of these function classes. 相似文献
4.
The set of all m × n Boolean matrices is denoted by $
\mathbb{M}
$
\mathbb{M}
m,n
. We call a matrix A ∈ $
\mathbb{M}
$
\mathbb{M}
m,n
regular if there is a matrix G ∈ $
\mathbb{M}
$
\mathbb{M}
n,m
such that AGA = A. In this paper, we study the problem of characterizing linear operators on $
\mathbb{M}
$
\mathbb{M}
m,n
that strongly preserve regular matrices. Consequently, we obtain that if min{m, n} ⩽ 2, then all operators on $
\mathbb{M}
$
\mathbb{M}
m,n
strongly preserve regular matrices, and if min{m, n} ⩾ 3, then an operator T on $
\mathbb{M}
$
\mathbb{M}
m,n
strongly preserves regular matrices if and only if there are invertible matrices U and V such that T(X) = UXV for all X ε $
\mathbb{M}
$
\mathbb{M}
m,n
, or m = n and T(X) = UX
T
V for all X ∈ $
\mathbb{M}
$
\mathbb{M}
n
. 相似文献
5.
Rajendra Bhatia 《印度理论与应用数学杂志》2010,41(1):99-111
Lipschitz continuity of the matrix absolute value |A| = (A*A)1/2 is studied. Let A and B be invertible, and let M
1 = max(‖A‖, ‖B‖), M
2 = max(‖A
−1‖, ‖B
−1‖). Then it is shown that
$
\left\| { \left| A \right| - \left| B \right| } \right\| \leqslant \left( {1 + log M_1 M_2 } \right) \left\| {A - B} \right\|
$
\left\| { \left| A \right| - \left| B \right| } \right\| \leqslant \left( {1 + log M_1 M_2 } \right) \left\| {A - B} \right\|
相似文献
6.
B. Wróbel 《Acta Mathematica Hungarica》2009,124(4):333-351
Imaginary powers associated to the Laguerre differential operator $
L_\alpha = - \Delta + |x|^2 + \sum _{i = 1}^d \frac{1}
{{x_i^2 }}(\alpha _i^2 - 1/4)
$
L_\alpha = - \Delta + |x|^2 + \sum _{i = 1}^d \frac{1}
{{x_i^2 }}(\alpha _i^2 - 1/4)
are investigated. It is proved that for every multi-index α = (α1,...α
d
) such that α
i
≧ −1/2, α
i
∉ (−1/2, 1/2), the imaginary powers $
\mathcal{L}_\alpha ^{ - i\gamma } ,\gamma \in \mathbb{R}
$
\mathcal{L}_\alpha ^{ - i\gamma } ,\gamma \in \mathbb{R}
, of a self-adjoint extension of L
α, are Calderón-Zygmund operators. Consequently, mapping properties of $
\mathcal{L}_\alpha ^{ - i\gamma }
$
\mathcal{L}_\alpha ^{ - i\gamma }
follow by the general theory. 相似文献
7.
In this article we investigate the p-rank of function fields in several good towers. To do this we first recall and establish some properties of the behaviour
of the p-rank under extensions. Then we compute the p-ranks of function fields in several optimal towers over a quadratic field $
\mathbb{F}_{q^2 }
$
\mathbb{F}_{q^2 }
, as well as for a specific good tower over a cubic field $
\mathbb{F}_{q^3 }
$
\mathbb{F}_{q^3 }
, which was introduced by Bassa, Garcia and Stichtenoth. 相似文献
8.
Natural bounded concentrators 总被引:1,自引:0,他引:1
Moshe Morgenstern 《Combinatorica》1995,15(1):111-122
We give the first known direct construction for linear families of bounded concentrators. The construction is explicit and
the results are simple natural bounded concentrators.
Let
be the field withq elements,g(x)∈F
q
[x] of degree greater than or equal to 2,
and
. LetI
nputs=H/A,O
utputs=H/B, and draw an edge betweenaA andbB iffaA∩bB≠ϕ. We prove that for everyq≥5 this graph is an
concentrator.
Part of this research was done while the author was at the department of Computer Science, The University of British Columbia,
Vancouver, B.C., Canada. 相似文献
9.
Mei-Chu Chang 《Combinatorica》2009,29(6):629-635
In this note, we use ‘classical’ methods to obtain sum-product theorems for subsets A⊂$
\mathbb{F}
$
\mathbb{F}
p
. 相似文献
10.
V. G. Puzarenko 《Siberian Advances in Mathematics》2010,20(2):128-154
We study some properties of a $
\mathfrak{c}
$
\mathfrak{c}
-universal semilattice $
\mathfrak{A}
$
\mathfrak{A}
with the cardinality of the continuum, i.e., of an upper semilattice of m-degrees. In particular, it is shown that the quotient semilattice of such a semilattice modulo any countable ideal will be
also $
\mathfrak{c}
$
\mathfrak{c}
-universal. In addition, there exists an isomorphism
$
\mathfrak{A}
$
\mathfrak{A}
such that $
{\mathfrak{A} \mathord{\left/
{\vphantom {\mathfrak{A} {\iota \left( \mathfrak{A} \right)}}} \right.
\kern-\nulldelimiterspace} {\iota \left( \mathfrak{A} \right)}}
$
{\mathfrak{A} \mathord{\left/
{\vphantom {\mathfrak{A} {\iota \left( \mathfrak{A} \right)}}} \right.
\kern-\nulldelimiterspace} {\iota \left( \mathfrak{A} \right)}}
will be also $
\mathfrak{c}
$
\mathfrak{c}
-universal. Furthermore, a property of the group of its automorphisms is obtained. To study properties of this semilattice,
the technique and methods of admissible sets are used. More exactly, it is shown that the semilattice of mΣ-degrees $
L_{m\Sigma }^{\mathbb{H}\mathbb{F}\left( S \right)}
$
L_{m\Sigma }^{\mathbb{H}\mathbb{F}\left( S \right)}
on the hereditarily finite superstructure $
\mathbb{H}\mathbb{F}
$
\mathbb{H}\mathbb{F}
(S) over a countable set S will be a $
\mathfrak{c}
$
\mathfrak{c}
-universal semilattice with the cardinality of the continuum. 相似文献
11.
David J. Grynkiewicz 《Israel Journal of Mathematics》2010,177(1):413-439
Let t ≥ 1, let A and B be finite, nonempty subsets of an abelian group G, and let $
A\mathop + \limits_i B
$
A\mathop + \limits_i B
denote all the elements c with at least i representations of the form c = a + b, with a ∈ A and b ∈ B. For |A|, |B| ≥ t, we show that either
|