3,721 research outputs found
Subexponential estimations in Shirshov's height theorem
In 1993 E. I. Zelmanov asked the following question in Dniester Notebook:
"Suppose that F_{2, m} is a 2-generated associative ring with the identity
x^m=0. Is it true, that the nilpotency degree of F_{2, m} has exponential
growth?"
We show that the nilpotency degree of l-generated associative algebra with
the identity x^d=0 is smaller than Psi(d,d,l), where Psi(n,d,l)=2^{18} l
(nd)^{3 log_3 (nd)+13}d^2. We give the definitive answer to E. I. Zelmanov by
this result. It is the consequence of one fact, which is based on combinatorics
of words. Let l, n and d>n be positive integers. Then all the words over
alphabet of cardinality l which length is greater than Psi(n,d,l) are either
n-divided or contain d-th power of subword, where a word W is n-divided, if it
can be represented in the following form W=W_0 W_1...W_n such that W_1 >'
W_2>'...>'W_n. The symbol >' means lexicographical order here. A. I. Shirshov
proved that the set of non n-divided words over alphabet of cardinality l has
bounded height h over the set Y consisting of all the words of degree <n.
Original Shirshov's estimation was just recursive, in 1982 double exponent was
obtained by A.G.Kolotov and in 1993 A.Ya.Belov obtained exponential estimation.
We show, that h<Phi(n,l), where Phi(n,l) = 2^{87} n^{12 log_3 n + 48} l. Our
proof uses Latyshev idea of Dilworth theorem application.Comment: In Russian, 22 pages. The current version of this paper differs from
the previous versions by the better estimation. English version of the
article is located at http://arxiv.org/abs/1207.298
Effect of perturbed flow on the transition from the supersonic laminar boundary layer to the turbulent
Results of experimental studies on the effect of various factors on the transition of a supersonic boundary layer are discussed. It is shown that in supersonic wind tunnels, a significant effect on the transition of the boundary layer on a model is exerted by the scale of acoustic perturbations, which is proportional to the boundary layer displacement thickness of the working section. Experimental data obtained over a wide range of variation of flow parameters in aerodynamically similar test installations with different dimensions of the working section are generalized by means of a correlation parameter based on the displacement thickness
Antiferromagnetic state in bilayer graphene
Motivated by the recent experiment of Velasco Jr. {\em et al.} [J. Velasco
Jr. {\em et al.}, Nat. Nanotechnology 7, {\bf 156} (2012)], we develop a
mean-field theory of the interaction-induced antiferromagnetic (AF) state in
bilayer graphene at charge neutrality point at arbitrary perpendicular magnetic
field B. We demonstrate that the AF state can persist at all . At higher
, the state continuously crosses over to the AF phase of the quantum
Hall ferromagnet, recently argued to be realized in the insulating
state. The mean-field quasiparticle gap is finite at B=0 and grows with
increasing B, becoming quasi-linear in the quantum Hall regime, in accord with
the reported behavior of the transport gap. By adjusting the two free
parameters of the model, we obtain a simultaneous quantitative agreement
between the experimental and theoretical values of the key parameters of the
gap dependence -- its zero-field value and slope at higher fields. Our findings
suggest that the insulating state observed in bilayer graphene in Ref. 1 is
antiferromagnetic (canted, once the Zeeman effect is taken into account) at all
magnetic fields.Comment: 5 pages, 3 figs; v3: published versio
- …