Constant time estimation of ranking statistics by analytic combinatorics

Abstract

International audienceWe consider i.i.d. increments (or jumps) X_i that are integers in [-c,...,+d], the partial sums S_j, and the discrete walks (j,S_j) with 1 <= j <= n. Late conditioning by a return of the walk to zero at time n provides discrete bridges that we note B_j with 1<= j <= n. We give in this extended abstract the asymptotic law in the central domain of the height max_{1<= j <= n}B_j of the bridges as n tends to infinity. As expected, this law converges to the Rayleigh law which is the law of the maximum of a standard Brownian bridge. In the case where c=1 (only one negative jump), we provide a full expansion of the asymptotic limit which improves upon the rate of convergence O(log(n)/sqrt(n)) given by Borisov (78) for lattice jumps; this applies in particular to the case where X_i is in {-1,+d}, in which case the expansion is expressible as a function of n, d and of the height of the bridge. Applying this expansion for X_i in {-1,d/c} gives an excellent approximation of the case X_i in {-d,+c} and provides in constant time an indicator used in ranking statistics; this indicator can be used for medical diagnosis and bioinformatics analysis (see Keller et al. (2007)) who compute it in time O(n min(c,d)) by use of dynamical programming)

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