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Let $X_n$ be random walk starting at $M< N$. At each step $X_n = k$ it goes up a unit step with probability $1-k/N$ otherwise it stays at its place. Do we have any nice interpretation of the asymptotic distribution of the time to reach $X_n = N$

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This is the coupon collector problem, with a started collection of $M$ coupons. The asymptotic is known to be $nH_n$, where $H_n$ is the $n^{th}$ harmonic number

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does it have any convergence to normal? –  marmar Dec 9 '12 at 0:23

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