The Mahonian Probability Distribution on Words Is Asymptotically Normal

The Mahonian statistic is the number of inversions in a permutation of a multiset with ai elements of type i, 1 ≤ i ≤ m. The counting function for this statistic is the q analog of the multinomial coefficient ( a1+···+am a1,...am ) , and the probability generating function is the normalization of the latter. We give two proofs that the distribution is asymptotically normal. The first is computer-assisted, based on the method of moments. The Maple package MahonianStat, available from the webpage of this article, can be used by the reader to perform experiments and calculations. Our second proof uses characteristic functions. We then take up the study of a local limit theorem to accompany our central limit theorem. Here our result is less general, and we must be content with a conjecture about further work. Our local limit theorem permits us to conclude that the coeffiecients of the q-multinomial are log-concave, provided one stays near the center (where the largest coefficients reside.)