Math 2.0

Potential theory Potential theory on fractals and metric spaces Theorem

Kigami's construction of Laplacians on post-critically finite fractals

Builds on Iterated function systemLaplacian matrixResistance formDirichlet form

Statement

Let K⊂ℝdK \subset \R^d be the attractor of an iterated function system of contracting similitudes F1,…,FNF_1, \dots, F_N, and assume that KK is connected and post-critically finite. Let V0⊂KV_0 \subset K be its finite boundary set, let Vm+1=⋃i=1NFi(Vm)V_{m+1} = \bigcup_{i=1}^N F_i(V_m), and for a word w=w1⋯wmw = w_1 \cdots w_m write Fw=Fw1∘⋯∘FwmF_w = F_{w_1} \circ \cdots \circ F_{w_m} and rw=rw1⋯rwmr_w = r_{w_1} \cdots r_{w_m}. Let ℰ0(u)=∑{p,q}⊂V0cpq(u(p)−u(q))2\mathcal{E}_0(u) = \sum_{\set{p, q} \subset V_0} c_{pq} (u(p) - u(q))^2 be the energy of an electrical network on V0V_0 with conductances cpq=cqp≥0c_{pq} = c_{qp} \ge 0 that connect V0V_0, and let r1,…,rN∈(0,1)r_1, \dots, r_N \in (0, 1). For u ⁣:Vm→ℝu \colon V_m \to \R set ℰm(u)=∑∣w∣=mrw−1 ℰ0(u∘Fw∣V0).\mathcal{E}_m(u) = \sum_{\abs{w} = m} r_w^{-1} \, \mathcal{E}_0(u \circ F_w|_{V_0}). Assume the renormalization condition: for every u ⁣:V0→ℝu \colon V_0 \to \R, the minimum of ℰ1(v)\mathcal{E}_1(v) over all v ⁣:V1→ℝv \colon V_1 \to \R with v∣V0=uv|_{V_0} = u equals ℰ0(u)\mathcal{E}_0(u). Then:

  1. For every function u ⁣:K→ℝu \colon K \to \R, the sequence ℰm(u∣Vm)\mathcal{E}_m(u|_{V_m}) is nondecreasing in mm; let ℰ(u)\mathcal{E}(u) be its limit in [0,∞][0, \infty].
  2. Let ℱ={u ⁣:K→ℝ:u continuous, ℰ(u)<∞}\mathcal{F} = \set{u \colon K \to \R : u \text{ continuous}, \ \mathcal{E}(u) < \infty}, and define ℰ(u,v)\mathcal{E}(u, v) from ℰ\mathcal{E} by polarization. Then (ℰ,ℱ)(\mathcal{E}, \mathcal{F}) is a resistance form on KK, and its resistance metric induces the usual topology of KK.
  3. For every finite Borel measure μ\mu on KK whose support is KK, (ℰ,ℱ)(\mathcal{E}, \mathcal{F}) is a local regular Dirichlet form on L2(K,μ)L^2(K, \mu).
  4. The Laplacian Δμ\Delta_\mu is defined by: Δμu=f\Delta_\mu u = f, with u∈ℱu \in \mathcal{F} and ff continuous, if ℰ(u,v)=−∫Kfv dμ\mathcal{E}(u, v) = -\int_K f v \, d\mu for every v∈ℱv \in \mathcal{F} that vanishes on V0V_0.

Remarks

The renormalization condition says that ℰ0\mathcal{E}_0 is the trace of ℰ1\mathcal{E}_1 on V0V_0, and by self-similarity ℰm\mathcal{E}_m is then the trace of ℰm+1\mathcal{E}_{m+1} on VmV_m for every mm. This gives the monotonicity in clause 1 and shows that each uu on VmV_m has a unique extension to Vm+1V_{m+1} with the same energy, the harmonic extension. The bound (u(x)−u(y))2≤R(x,y) ℰ(u)(u(x) - u(y))^2 \le R(x, y) \, \mathcal{E}(u) with a resistance RR that is small on small cells gives the continuity of functions of finite energy; this uses ri<1r_i < 1. For the Sierpiński gasket, with ℰ0\mathcal{E}_0 the unit-conductance triangle and all ri=3/5r_i = 3/5, the renormalization condition is the computation of the "1/51/5–2/52/5 rule", as in Laplacian on the Sierpiński gasket. Finding ℰ0\mathcal{E}_0 and rir_i that satisfy the renormalization condition is a nonlinear fixed-point problem, and the theorem takes a solution as given. A pair (ℰ0,(ri))(\mathcal{E}_0, (r_i)) that satisfies the renormalization condition is called a harmonic structure, and it is called regular when all ri<1r_i < 1. The construction is due to Kigami.

Going deeper

For a student

The result turns the analysis of a fractal into linear algebra on a sequence of finite graphs, each step governed by the same Laplacian matrix. Harmonic functions are determined by their values on V0V_0 and are computed level by level through fixed harmonic extension matrices. Once the Laplacian exists, one studies its spectrum, the heat equation ∂tu=Δμu\partial_t u = \Delta_\mu u and its heat kernel, and the diffusion process attached to the Dirichlet form by the Fukushima correspondence. The resistance metric is the natural metric for these estimates. The approach complements the probabilistic one, in which Brownian motion on the gasket is built as a limit of random walks on the graphs Γm\Gamma_m.

Reading path

To understand this page from the ground up, read these 5 pages in order. The foundations come first.

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  1. Dirichlet form concept
  2. Iterated function system concept
  3. Laplacian matrix concept
  4. Resistance form concept
  5. Kigami's construction of Laplacians on post-critically finite fractals statement

Further reading

  1. Web https://en.wikipedia.org/wiki/Analysis_on_fractals
About this page
TagVXB4G47Z3T
URIm2://VXB4G47Z3T
Kindstatement, theorem
WritingW0 stub
MSC31E05, 28A80, 31C25
StatusThis copy of the site has no build data. Run m2 status VXB4G47Z3T.
AboutAnalysis on fractals, Resistance form
UsesIterated function system, Laplacian matrix, Resistance form, Dirichlet form

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