# Zaslavsky web map

Post-publication activity

Curator: George Zaslavsky

The stochastic web is a thin net of fibers of finite width in the phase space of a Hamiltonian system with chaotic trajectories within the web and with regular dynamics outside one, at least in the web's vicinity. A paradigm example of the stochastic web is the Arnold web (Arnold, 1964; Arnold et al., 2006) along which a particle can perform unbounded Arnold diffusion. Such a web exists for the number of degrees of freedom $$N>2$$ and non-degeneracy condition for the Hessian$Hess H_0 \equiv | \partial^2 H_0 / \partial I \partial I | \neq0\ ,$ where $$H_0=H_0(I)$$ is unperturbed Hamiltonian, and action $$I \in {\mathbb R}^N\ .$$

## Stochastic web map

The stochastic web map, known as Zaslavsky web map, occurs in Hamiltonian systems with $$N>1$$ satisfying the degeneracy condition $$Hess H_0=0$$ (Zaslavsky et al., 1986; Zaslavsky et al., 1991).

The web map is generated by a periodically kicked linear oscillator with the Hamiltonian: $\tag{1} H=(1/2)(\dot{x}^2+\omega_0^2x^2)-(\omega_0K/T) \cos x \sum_{m=-\infty}^{\infty} \delta(t/T-m)$

where $$T$$ is the period and $$K$$ is the intensity of the kicks. The corresponding map $$\hat{T_{\alpha}}$$ connects the dimensionless coordinates $$(u=\dot{x}/\omega_0, v=-x)$$ between two successive kicks: $u_{n+1}= (u_n+K\sin v_n)\cos \alpha + v_n \sin \alpha$ $$\tag{2} \hat{T_{\alpha}}\ :$$

$v_{n+1}= -(u_n+K\sin v_n)\sin \alpha + v_n \cos \alpha.$ The most interesting case occurs when the oscillator and the kicks are in resonance$\alpha = \alpha_q = 2 \pi /q\ ,$ $$q \in {\mathbb N}\ ,$$ $$q \geq 3\ .$$ Then the stochastic web tiles the phase plane $$(u,v)$$ (see Figs. 1-5).  Figure 1: Thin stochastic web for $$q=4\ ,$$ $$K=1.5$$ is filled by the trajectory of the map $$\hat{T_{\alpha}}\ .$$ There are invariant curves and isolated from the web stochastic layers inside the cells created by the web.  Figure 2: Magnification of the stochastic web in Fig.1 shows that area of the web is non-uniformly filled and has very complex pattern with islands and subislands.  Figure 3: Stochastic web for $$q=6\ ,$$ $$K=2$$ by the only trajectory of the map $$\hat{T_{\alpha}}\ .$$ Nonuniform density of points along the web represents a result of the random walk process along the web for a finite time. Figure 4: Animation of the random walk process created by the only trajectory of the map $$\hat{T_{\alpha}}$$ for $$q=5\ ,$$ $$K=0.8\ .$$ The 5-fold symmetry web emerges in the phase space as a result of the random walk. Figure 5: The same as in Fig.4 but for $$q=8\ ,$$ $$K=.8108745\ .$$  Figure 6: "Thick isolines" of the web skeleton Hamiltonian $$H_q(u,v)\ ,$$ for $$q=4\ .$$ Different colors correspond to the sets of points $$(u,v)$$ that belong to different layers of $$H_q \in (E_m-\Delta E/2, E_m+\Delta E/2)\ .$$  Figure 7: The web skeleton , as in Fig.6, but for $$q=3\ .$$  Figure 8: The web skeleton , as in Fig.6, but for $$q=5\ .$$ The pattern is similar to the 5-fold symmetry 2-dimensional quasicrystal.  Figure 9: The web skeleton , as in Fig.6, but for $$q=8\ .$$ Figure 10: Animation of a particle trajectory along the 4-fold symmetry stochastic web for $$K=6.349$$ demonstrates anomalous (non-Gaussian) diffusion with Levy flights.

Some properties of the stochastic webs are: (a) The stochastic web can appear for $$\alpha$$ rational or fairly close to a rational for finite $$K\ ;$$ (b)The structure of the web can be characterized by the web skeleton, i.e. by some net of channels filled by any trajectory that starts within the web. Evidently, the skeleton is an invariant, i.e. its shape doesn't depend on time. (c)The web structure, i.e. its skeleton, has a symmetry of the crystalline type for $$q \in \{q_c\} \equiv (3,4,6)$$ and of the quasicrystal type if $$q \not\in \{q_c\}\ ;$$ (d) The stochastic web exists for an arbitrary small $$K$$ if $$q \in \{q_c\}$$ and the size of the meshes is independent of $$K\ .$$ It is conjectured that for $$q \not\in \{q_c\}$$ the web exists for any small $$K$$ but the smaller is $$K\ ,$$ the larger are the web's meshes.

## The Web Skeleton

The invariant structure of the stochastic web can be obtained by averaging of (1) over the period $$T\ .$$ Then, $$H=H(u,v,t) \rightarrow H_q(u,v)$$ and $\tag{3} H_q(u,v)=-(K/q)\ \sum_{j=1}^{q} \cos(\rho \cdot e_j)$

where $$\rho =(u,v)$$ and $$e_j=(\cos (2\pi j/q)-\sin (2\pi j/q))$$ are the unit vectors that form a regular $$q$$-star. Isolines generated by (3) (satisfying $$H_q=$$const) are shown in Figs 6-9. The web skeleton can be considered as a "thick isoline" obtained from (3) when $$H_q \in (E_m-\Delta E/2, E_m+\Delta E/2)\ ,$$ $$\Delta E$$ is small, and $$E_m$$ corresponds to the value of $$H_q$$ for which the distribution function of the number of saddle points as a function of energy has a maximum. For $$q \in \{q_c\}$$ such skeleton exists even if $$\Delta E \rightarrow 0\ .$$ The web skeleton can be used as a stencil for different kinds of art patterns such as 5-fold Penrose tiling. Different oriental ornaments with 5-fold symmetric stencils were found in decorations of 11-14 centuries in Iran, Granada, Cordoba (see Zaslavsky et al., 1991 and references therein).

The map $$\hat{T_{\alpha}}$$ for $$\alpha=\alpha_q$$ can be considered as a dynamical generator of the $$q$$-fold symmetry for arbitrary integer $$q\ .$$ It appeared first in the description of a charged particle dynamics in a constant magnetic wave packet. In general, the map (2) is an alternative to the Chirikov-Taylor standard map since $$H_0=(1/2)(\dot{x}^2+\omega_0^2x^2)$$ is degenerate and thus KAM theory cannot be directly applied to (1).

Important developments on the stochastic web map were obtained in (Pekarsky and Rom-Kedar, 1997; Dana and Amit, 1995; Lowenstein, 1993 and 1995). Particles dynamics along the stochastic web generated by the map (2) is diffusive and unbounded contrary to the Arnold web along which the unbounded diffusion can be only for $$N>2\ .$$ The diffusion is, in general, anomalous and can be described by fractional kinetics (Zaslavsky, 2005). Particularly, it can be superdiffusion. An example for superdiffusion with $$q=4$$ ($$\hat{T_{\pi /2}}: u_{n+1}=v_n, v_{n+1}=-u_n-K \sin (v_n)$$) is shown in Fig. 10.