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Upper transition point for percolation on the enhanced binary tree: A sharpened lower bound

Hyperbolic structures are obtained by tiling a hyperbolic surface with negative Gaussian curvature. These structures generally exhibit two percolation transitions: a system-wide connection can be established at a certain occupation probability $p=p_{c1}$ and there emerges a unique giant cluster at $p_{c2} > p_{c1}$. There have been debates about locating the upper transition point of a prototypical hyperbolic structure called the enhanced binary tree (EBT), which is constructed by adding loops to a binary tree. This work presents its lower bound as $p_{c2} \gtrsim 0.55$ by using phenomenological renormalization-group methods and discusses some solvable models related to the EBT.