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 $Q_n(t)$?
Best Answer
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.