The coupon collector’s problem revisited: generalizing the double Dixie cup problem of Newman and Shepp

Department of Mathematics, National Technical University of Athens, Zografou Campus, 15780 Athens, Greece.
aris.doumas@hotmail.com; papanico@math.ntua.gr

Received: 1 September 2015
Revised: 1 June 2016
Accepted: 8 June 2016

Abstract

The “double Dixie cup problem” of [D.J. Newman and L. Shepp, Amer. Math. Monthly 67 (1960) 58–61] is a well-known variant of the coupon collector’s problem, where the object of study is the number T_m(N) of coupons that a collector has to buy in order to complete m sets of all N existing different coupons. More precisely, the problem is to determine the asymptotics of the expectation (and the variance) of T_m(N), as well as its limit distribution, as the number N of different coupons becomes arbitrarily large. The classical case of the problem, namely the case of equal coupon probabilities, is here extended to the general case, where the probabilities of the selected coupons are unequal. In the beginning of the article we give a brief review of the formulas for the moments and the moment generating function of the random variable T_m(N). Then, we develop techniques of computing the asymptotics of the first and the second moment of T_m(N) (our techniques apply to the higher moments of T_m(N) as well). From these asymptotic formulas we obtain the leading behavior of the variance V [ T_m(N) ] as N → ∞. Finally, based on the asymptotics of E [ T_m(N) ] and V [ T_m(N) ] we obtain the limit distribution of the random variable T_m(N) for large classes of coupon probabilities. As it turns out, in many cases, albeit not always, T_m(N) (appropriately normalized) converges in distribution to a Gumbel random variable. Our results on the limit distribution of T_m(N) generalize a well-known result of [P. Erdős and A. Rényi, Magyar. Tud. Akad. Mat. Kutató Int. Közl. 6 (1961) 215–220] regarding the limit distribution of T_m(N) for the case of equal coupon probabilities.