Some large deviations in Kingman's coalescent

Kingman's coalescent is a random tree that arises from classical population genetic models such as the Moran model. The individuals alive in these models correspond to the leaves in the tree and the following two laws of large numbers concerning the structure of the tree-top are well-known: (i) The (shortest) distance, denoted by $T_n$, from the tree-top to the level when there are $n$ lines in the tree satisfies $nT_n \xrightarrow{n\to\infty} 2$ almost surely; (ii) At time $T_n$, the population is naturally partitioned in exactly $n$ families where individuals belong to the same family if they have a common ancestor at time $T_n$ in the past. If $F_{i,n}$ denotes the size of the $i$th family, then $n(F_{1,n}^2 + \cdots + F_{n,n}^2) \xrightarrow{n\to \infty}2$ almost surely. For both laws of large numbers we prove corresponding large deviations results. For (i), the rate of the large deviations is $n$ and we can give the rate function explicitly. For (ii), the rate is $n$ for downwards deviations and $\sqrt n$ for upwards deviations. For both cases we give the exact rate function.

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