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Leaving the unit neighbourhood on a triangular lattice

Question

A walker sits at a vertex of the infinite triangular lattice made of unit equilateral triangles, so each vertex has 66 neighbours. On every move the walker steps along one of the 66 edges at its current vertex, chosen uniformly and independently of the past. What is the probability that at some point during the first 55 moves the walker is at distance strictly greater than 11 from where it began?

Short answer

95108\tfrac{95}{108}. Compute the complement: staying within distance 1 means being at the start or at one of its 6 neighbours. From a neighbour, 11 edge leads back, 22 lead to other neighbours and 33 lead outside. Tracking the probabilities of being at the start or on the ring gives a total of 281+31324=13108\tfrac{2}{81}+\tfrac{31}{324}=\tfrac{13}{108} of staying inside after 5 moves, so the answer is 1−131081-\tfrac{13}{108}.

Adapted from QuantQuestions.io; question and answer rewritten by SelfMadeQuant.com.

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