Abstract
We consider the operator \(H=L+V\) that is a perturbation of the Taibleson–Vladimirov operator \(L=\mathfrak{D}^\alpha\) by a potential \(V(x)=b\|x\|^{-\alpha}\), where \(\alpha>0\) and \(b\geq b_*\). We prove that the operator \(H\) is closable and its minimal closure is a nonnegative definite self-adjoint operator (where the critical value \(b_*\) depends on \(\alpha\)). While the operator \(H\) is nonnegative definite, the potential \(V(x)\) may well take negative values as \(b_*<0\) for all \(0<\alpha<1\). The equation \(Hu=v\) admits a Green function \(g_H(x,y)\), that is, the integral kernel of the operator \(H^{-1}\). We obtain sharp lower and upper bounds on the ratio of the Green functions \(g_H(x,y)\) and \(g_L(x,y)\).
Notes
A Markov process \(\{X(t),P_x\}\) with state space \( {\mathcal X} \) is called a pure jump process if, starting from any point \(x\in {\mathcal X} \), it has all sample paths constant except for isolated jumps, and is right-continuous. The basic data which define the process are (i) a function \(0<\lambda(x)<\infty\) and (ii) a Markov kernel \( {\mathcal U} (x,dy)\) satisfying the equality \( {\mathcal U} (x,\{x\})=0\). Its Laplacian (i.e., minus Markov generator) has the form
$$Lf(x)=\intop_{ {\mathcal X} }\bigl(f(x)-f(y)\bigr) \lambda(x) {\mathcal U} (x,dy).$$Intuitively a particle starting from \(x\) remains there for an exponentially distributed time with parameter \(\lambda(x)\), after which it “jumps” to a new position \(x'\) according to the distribution \( {\mathcal U} (x,\cdot \kern1pt )\), and so on.
In the case where \(\Phi(\tau)\) is a Bernstein function, the relation \(L_{\Phi}=\Phi(L_{\text{Id}})\) has been studied in the well-known Bochner subordination theory (see [19]).
This relation must be compared with the Green function estimates for Schrödinger operators on Riemannian manifolds (see [22]).
The following counterpart of Theorem 4.5 is in order: Let \(X^{ {\mathcal H} }\) and \(X^{ {\mathcal L} }\) be the Hunt processes associated with the Dirichlet forms \(Q_{ {\mathcal H} }\) and \(Q_{ {\mathcal L} }\), respectively. According to [20, Theorem 5.5.2 and Example 5.5.1], their paths are related by the random time change \(X_t^{ {\mathcal H} } =X_{\tau_t}^{ {\mathcal L} }\), where \(\tau_t=\inf\{s>0 \colon\, A_t>t\}\) and \(A_t=\intop_0^th(X_s^{ {\mathcal H} })\,ds\) is a positive continuous additive functional. It follows, in particular, that Dynkin’s characteristic operators for these processes are related by the equation \((- {\mathcal H} u)(x)=(- {\mathcal L} u)(x)/h(x)\). We are going to use this fact in the next subsections to solve the equation \(Hu=v\).
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Funding
A.B. and A.G. were funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), grant no. SFB 1283/2 2021 – 317210226. The work of S.M. was supported by the Russian Science Foundation under grant no. 17-11-01098, https://rscf.ru/en/project/17-11-01098/.
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Published in Russian in Trudy Matematicheskogo Instituta imeni V.A. Steklova, 2023, Vol. 323, pp. 17–52 https://doi.org/10.4213/tm4356.
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Bendikov, A., Grigor’yan, A. & Molchanov, S. Hierarchical Schrödinger Operators with Singular Potentials. Proc. Steklov Inst. Math. 323, 12–46 (2023). https://doi.org/10.1134/S0081543823050024
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DOI: https://doi.org/10.1134/S0081543823050024