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If $\lambda=(\lambda_1\geq\lambda_2\geq\dots\geq\lambda_k)\vdash n$ is an integer partition of $n$ then $\lambda_1$ is its largest part and $k$ is its length, $\ell(\lambda)$.

Define the statistic $c_n(\lambda)=\max\{\lambda_1,\ell(\lambda)\}$ for the above partition. Also consider the polynomial (in $t$), $$Q_n(t)=\sum_{\lambda\vdash n}t^{c_n(\lambda)}.$$ Here are some examples:

 t 2 2 t 3 2 2 t + t 4 3 2 2 t + 2 t + t 5 4 3 2 t + 2 t + 3 t 6 5 4 3 2 t + 2 t + 4 t + 3 t 7 6 5 4 3 2 t + 2 t + 4 t + 5 t + 2 t 8 7 6 5 4 3 2 t + 2 t + 4 t + 6 t + 7 t + t 9 8 7 6 5 4 3 2 t + 2 t + 4 t + 6 t + 9 t + 6 t + t 10 9 8 7 6 5 4 2 t + 2 t + 4 t + 6 t + 10 t + 11 t + 7 t 

QUESTION 1. It appears that the coefficients of $Q_n(t)$, read from left to right, are twice the partition numbers $1,1,2,3,5,7,11,15,\dots$, up to (at least) the middle term. Is this true?

QUESTION 2. Is there a generating function for the polynomials $Q_n(t)$?

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  • $\begingroup$ Yes, remove the first part (or the first column). $\endgroup$ Commented Jan 18, 2023 at 22:24
  • $\begingroup$ $P_n = Q_n\dots$? $\endgroup$ Commented Jan 19, 2023 at 14:52
  • $\begingroup$ Sorry for the typo. Yes, $Q_n$. $\endgroup$ Commented Jan 19, 2023 at 16:20

1 Answer 1

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Your $c_n(\lambda)$ is closely related to what Billey, Konvalinka, Swanson 2020 call the aft of a partition: $$ \text{aft}(\lambda) = | \lambda | - \max\{\lambda_1, \lambda'_1\}$$ where $\lambda'$ is the conjugate of $\lambda$, so $\lambda'_1 = \ell(\lambda)$.

Following up on Martin Rubey's comment, your Question 1 is true: For $c_n(\lambda) > n/2$, we have $\lambda \ne \lambda'$ since $\lambda_1 = \lambda'_1$ would make $2\lambda_1 > n$, a contradiction. Therefore, $\lambda$ is not self-conjugate and each partition $\mu \vdash (n - \lambda_1)$ is counted twice in the coefficient of $t^{\lambda_1}$, once for the partition of $n$ consisting of $\lambda_1$ followed by $\mu$ and once for the conjugate of that partition of $n$.

For your Question 2, a generating function in terms of Gaussian binomial coefficients is given in the related OEIS entry A338621 submitted by Swanson, one of the article authors.

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