We study the Dyson rank function N (r, t; n), the number of partitions with rank congruent to r modulo t. We first show that it is monotonic in n, and then show that it equidistributed as n → ∞. Using this result we prove a conjecture of Hou and Jagadeeson on the convexity of N (r, t; n).
acknowledgementsThe author would like to thank Kathrin Bringmann for helpful comments on previous versions of the paper, as well as Chris Jennings-Shaffer for useful conversations. The author would also like to thank the referee for many helpful comments.Recently, in [1] Ono and Bessenrodt showed that the partition function satisfies the following convexity result. If a, b ≥ 1 and a + b ≥ 9 then p(a)p(b) > p(a + b).A natural question to ask is then: does N (r, t; n) satisfy a similar property? In [9] Hou and Jagadeesan provide an answer if t = 3. They showed that for 0 ≤ r ≤ 2 we have N (r, 3; a)N (r, 3; b) > N (r, 3; a + b) for all a, b larger than some specific bound. Further, at the end of the same paper, the authors offer the following conjecture on a more general convexity result. Conjecture 1.2. For 0 ≤ r < t and t ≥ 2 then N (r, t; a)N (r, t; b) > N (r, t; a + b) for sufficiently large a and b.As a simple consequence of Theorem 1.1 we prove the following theorem. Theorem 1.3. Conjecture 1.2 is true.Remark. We note that unlike in [9] our proof of Theorem 1.3 does not give an explicit lower bound on a and b. To yield such a bound one could employ similar techniques to those in [9], relying on the asymptotics found in [2]. However, since [2] gives results only for odd t one could only find such bounds directly for odd t. Further, to find an explicit bound for general t is a difficult problem.