Eventual log-concavity of the coefficients of the q-Catalan numbers

Let Cn(q)=k=0n(n1)mn(k)qkC_n(q)=\sum_{k=0}^{n(n-1)}m_n(k)q^k denote the qq-Catalan numbers and let mn(k)m_n(k) be their coefficients. Log-concavity conjecture. There exists an integer tt such that, when nn is sufficiently large, the sequence

{mn(t),,mn(n(n1)t)}\{m_n(t),\ldots,m_n(n(n-1)-t)\}

is log-concave, namely

(mn(k))2mn(k+1)mn(k1)\left(m_n(k)\right)^2\geq m_n(k+1)m_n(k-1)

for t+1kn(n2)t1t+1\leq k\leq n(n-2)-t-1. Moreover, the minimum value of tt seems to be 7575. This is presented as a stronger conjecture suggested by numerical evidence for n>70n>70; it remains open.

Sources & referencesView supporting material

Primary source

William Y. C. Chen, Carol J. Wang and Larry X. W. Wang, “The Limiting Distribution of the Coefficients of the q-Catalan Numbers”, arXiv:0708.2574 (2007).

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