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1.
Mathieu Florence 《Inventiones Mathematicae》2008,171(1):175-189
Let p be a prime number, let K be a field of characteristic not p, containing the p-th roots of unity, and let r≥1 be an integer. We compute the essential dimension of ℤ/p
r
ℤ over K (Theorem 4.1). In particular,
i) We have edℚ(ℤ/8ℤ)=4, a result which was conjectured by Buhler and Reichstein in 1995 (unpublished).
ii) We have edℚ(ℤ/p
r
ℤ)≥p
r-1. 相似文献
2.
We present a short and direct proof (based on the Pontryagin-Thom construction) of the following Pontryagin-Steenrod-Wu theorem:
(a) LetM be a connected orientable closed smooth (n + 1)-manifold,n≥3. Define the degree map deg: π
n
(M) →H
n
(M; ℤ) by the formula degf =f*[S
n
], where [S
n
] εH
n
(M; ℤ) is the fundamental class. The degree map is bijective, if there existsβ εH
2(M, ℤ/2ℤ) such thatβ ·w
2(M) ε 0. If suchβ does not exist, then deg is a 2-1 map; and (b) LetM be an orientable closed smooth (n+2)-manifold,n≥3. An elementα lies in the image of the degree map if and only ifρ
2
α ·w
2(M)=0, whereρ
2: ℤ → ℤ/2ℤ is reduction modulo 2. 相似文献
3.
Matija Cencelj Dušan Repovš Mikhail B. Skopenkov 《Proceedings Mathematical Sciences》2007,117(3):301-306
We present a short and complete proof of the following Pontryagin theorem, whose original proof was complicated and has never
been published in detail. Let M be a connected oriented closed smooth 3-manifold, L
1(M) be the set of framed links in M up to a framed cobordism, and deg: L
1(M) → H
1(M; ℤ) be the map taking a framed link to its homology class. Then for each α ∈ H
1(M; ℤ) there is a one-to-one correspondence between the set deg−1
α and the group ℤ2d(α), where d(α) is the divisibility of the projection of α to the free part of
H
1(M; ℤ). 相似文献
4.
Klaus Schmidt 《Israel Journal of Mathematics》1995,90(1-3):295-300
Letp>1 be prime, and letY ⊂X=(ℤ/pℤ)ℤ
2) be an infinite, closed, shift-invariant subgroup with the following properties: the restriction toY of the shift-actionσ of ℤ2 onX is mixing with respect to the Haar measureλ
Y
ofY, and every closed, shift-invariant subgroupZ ⊂Y is finite. We prove that every sufficiently mixing, non-atomic, shift-invariant probability measureμ onY is equal toλ
Y
.
The author would like to thank the Department of Mathematics, University of Vienna, for hospitality while this work was done. 相似文献
5.
Consider the Cauchy problem ∂u(x, t)/∂t = ℋu(x, t) (x∈ℤd, t≥ 0) with initial condition u(x, 0) ≡ 1 and with ℋ the Anderson Hamiltonian ℋ = κΔ + ξ. Here Δ is the discrete Laplacian, κ∈ (0, ∞) is a diffusion constant,
and ξ = {ξ(x): x∈ℤ
d
} is an i.i.d.random field taking values in ℝ. G?rtner and Molchanov (1990) have shown that if the law of ξ(0) is nondegenerate,
then the solution u is asymptotically intermittent.
In the present paper we study the structure of the intermittent peaks for the special case where the law of ξ(0) is (in the
vicinity of) the double exponential Prob(ξ(0) > s) = exp[−e
s
/θ] (s∈ℝ). Here θ∈ (0, ∞) is a parameter that can be thought of as measuring the degree of disorder in the ξ-field. Our main result
is that, for fixed x, y∈ℤ
d
and t→∈, the correlation coefficient of u(x, t) and u(y, t) converges to ∥w
ρ∥−2
ℓ2Σz ∈ℤd
w
ρ(x+z)w
ρ(y+z). In this expression, ρ = θ/κ while w
ρ:ℤd→ℝ+ is given by w
ρ = (v
ρ)⊗
d
with v
ρ: ℤ→ℝ+ the unique centered ground state (i.e., the solution in ℓ2(ℤ) with minimal l
2-norm) of the 1-dimensional nonlinear equation Δv + 2ρv log v = 0. The uniqueness of the ground state is actually proved only for large ρ, but is conjectured to hold for any ρ∈ (0, ∞).
empty
It turns out that if the right tail of the law of ξ(0) is thicker (or thinner) than the double exponential, then the correlation
coefficient of u(x, t) and u(y, t) converges to δ
x, y
(resp.the constant function 1). Thus, the double exponential family is the critical class exhibiting a nondegenerate correlation
structure.
Received: 5 March 1997 / Revised version: 21 September 1998 相似文献
6.
T. O. Banach 《Mathematical Notes》1998,64(3):295-302
For each Abelian groupG, a cardinal invariant χ(G) is introduced and its properties are studied. In the special caseG = ℤ
n
, the cardinalχ(ℤ
n
) is equal to the minimal cardinality of an essential subset of ℤ
n
, i.e., a of a subsetA ⊂ ℤ
n
such that, for any coloring of the group ℤ
n
inn colors, there exists an infinite one-color subset that is symmetric with respect to some pointα ofA. The estimaten(
n + l)/2 ≤χ(ℤ
n
) < 2n is proved for alln and the relationχ(ℤ
n
) =n(n + 1)/2 forn ≤ 3. The structure of essential subsets of cardinalityχ(ℤ
n
) in ℤ
n
is completely described forn ≤ 3.
Translated fromMatematicheskie Zametki, Vol. 64, No. 3, pp. 341–350, September, 1998. 相似文献
7.
A. E. Troitskaya 《Journal of Mathematical Sciences》2009,159(6):879-893
Assume that Δ and Π are representations of the group ℤ2 by operators on the space L
2(X, μ) that are induced by measure-preserving automorphisms, and for some d, the representations Δ⨂d
and Π⨂d
are conjugate to each other, Δ(ℤ2
\(0, 0)) consists of weakly mixing operators, and there is a weak limit (over some subsequence in ℤ2 of operators from Δ(ℤ2)) which is equal to a nontrivial, convex linear combination of elements of Δ(ℤ2) and of the projection onto constant functions. We prove that in this case, Δ and Π are also conjugate to each other.
Translated from Fundamentalnaya i Prikladnaya Matematika, Vol. 13, No. 8, pp. 193–212, 2007. 相似文献
8.
We prove that if the existence of a supercompact cardinal is consistent with ZFC, then it is consistent with ZFC that the
p-rank of Ext
ℤ(G, ℤ) is as large as possible for every prime p and for any torsion-free Abelian group G. Moreover, given an uncountable
strong limit cardinal μ of countable cofinality and a partition of Π (the set of primes) into two disjoint subsets Π0 and Π1, we show that in some model which is very close to ZFC, there is an almost free Abelian group G of size 2μ = μ+ such that the p-rank of Ext
ℤ(G, ℤ) equals 2μ = μ+ for every p ∈ Π0 and 0 otherwise, that is, for p ∈ Π1.
Number 874 in Shelah’s list of publications. Supported by the German-Israeli Foundation for Scientific Research & Development
project No. I-706-54.6/2001.
Supported by a grant from the German Research Foundation DFG.
__________
Translated from Algebra i Logika, Vol. 46, No. 3, pp. 369–397, May–June, 2007. 相似文献
9.
Abdelmalek Azizi 《Rendiconti del Circolo Matematico di Palermo》1999,48(1):71-92
Let ℕ,i=√−1,k=ℚ(√d,i) andC
2 the 2-part of the class group ofk. Our goal is to determine alld such thatC
2⋍ℤ/2ℤ×ℤ/2ℤ.
Soientd∈ℕ,i=√−1,k=ℚ(√d,i), etC
2 la 2-partie du groupe de classes dek. On s'intéresse à déterminer tous lesd tel queC
2⋍ℤ/2ℤ×ℤ/2ℤ.
相似文献
10.
We study some properties of sets of differences of dense sets in ℤ2 and ℤ3 and their interplay with Bohr neighbourhoods in ℤ. We obtain, inter alia, the following results.
(i) | If E ⊂ ℤ2, $
\bar d
$
\bar d
(E) > 0 and p
i
, q
i
∈ ℤ[x], i = 1, ..., m satisfy p
i
(0) = q
i
(0) = 0, then there exists B ⊂ ℤ such that $
\bar d
$
\bar d
(B) > 0 and
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