Abstract
We study branching diffusions in a bounded domain D of \(\mathbb {R}^d\) in which particles are killed upon hitting the boundary \(\partial D\). It is known that any such process undergoes a phase transition when the branching rate \(\beta \) exceeds a critical value: a multiple of the first eigenvalue of the generator of the diffusion. We investigate the system at criticality and show that the associated genealogical tree, when the process is conditioned to survive for a long time, converges to Aldous’ Continuum Random Tree under appropriate rescaling. The result holds under only a mild assumption on the domain, and is valid for all branching mechanisms with finite variance, and a general class of diffusions.
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Notes
We say that \(D\subset \mathbb {R}^d\) is a {Lipschitz / \(C^k\) / \(C^{k,\alpha }\)} domain (for \(k\in \mathbb {Z}_{\ge 0}\) and \(\alpha \in [0,1]\)) if, at each point \(x_0\in \partial D\), there exists \(r>0\) and a {Lipschitz / \(C^k\) / \(C^{k,\alpha }\)} function \(\gamma :\mathbb {R}^{d-1}\rightarrow \mathbb {R}\) such that relabelling and reorienting axes as necessary, \(D\cap B(x_0,r)=\{x\in B(x_0,r): x^d>\gamma (x^1,\ldots , x^{d-1})\}\).
We define \(H_0^1(D)\) to be the closure of \(C_c^\infty (D)\) (the space of infinitely differentiable functions with compact support strictly inside D) with respect to the norm \(\Vert u||_{H^1(D)}:=\Vert u\Vert _{L^2} +\sum _{i=1}^d \Vert D^{x_i}(u)\Vert _{L^2}\), where \(D^{x_i}u\) is the ith partial derivative of u in the weak sense.
Indeed, since \(\hat{p}(t):=\sup \{p(u,\varepsilon ): u\ge t\}\) converges monotonically to 0 as \(t\rightarrow \infty \), we can choose \(g(t,\varepsilon ) \le \sqrt{t}\) but still converging to \(\infty \), such that \(g(t,\varepsilon )\hat{p}(\sqrt{t})\rightarrow 0\) as \(t\rightarrow \infty \). Then since \(p(t/g(t,\varepsilon ),\varepsilon )\le \hat{p}( t/g(t,\varepsilon ))\le \hat{p}(\sqrt{t})\), the function g will satisfy (5.9).
That is, a point on the tree corresponding to one individual dying, and being replaced with a strictly positive number of offspring.
For example, if w is the oldest younger sibling of \(v_t\) (so \(v_t=uj\) and \(w=u(j+1)\) for some \(j\ge 1\)) then \(\sigma (w)\) would be \((t-s)\) minus the minimum of \((t-s)\) and the total length of the subtree rooted at \(v_s\).
In fact, since \((\bar{S}_t^n)_t\) may have jumps, we also need to verify an extra condition in order to apply the functional central limit theorem. However this condition, see [32, § VIII, Eq.(3.23)], is simply that we have, for every \(t>0\),
$$\begin{aligned} \frac{\lambda }{m-1}\int _0^{t} \varphi (V_{s-}(w))^2\mathbb {E}\big ((A-1)^2 \mathbb {1}_{\{(A-1)^2 \varphi (V_{s-}(w))^2>n\varepsilon \}}\big ) \, ds \rightarrow 0 \end{aligned}$$almost surely as \(n\rightarrow \infty \) (where the expectation \(\mathbb {E}\) is only over A). Since A has finite variance, this does indeed hold.
Interpreting \(\phi (D_t^H)=\phi (((D_t^H)_{ij})_{1\le i<j\le k})\).
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Acknowledgements
I would particularly like to thank Nathanaël Berestycki, for suggesting this problem and for many helpful discussions. I am also grateful to the anonymous referee for numerous useful comments and suggestions.
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Powell, E. An invariance principle for branching diffusions in bounded domains. Probab. Theory Relat. Fields 173, 999–1062 (2019). https://doi.org/10.1007/s00440-018-0847-8
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DOI: https://doi.org/10.1007/s00440-018-0847-8