What is the formula for $\mathcal P_{n}^{k} (a_{1}, a_{2}, ...)$, defined by Peter Luschny?

Question

Recently, I was reading a blog post called The P-transform by Peter Luschny, where the following formulas are given:

\begin{align*} (-1)^k\frac{n!}{k!}\mathcal P^k_n\left(1, \frac1 2, \frac2 3, \dotsc\right) & = \genfrac[]{0pt}{}n k \\ (-1)^k\frac{n!}{k!}\mathcal P^k_n(1, 1, 1, \dotsc) & = \genfrac\lvert\rvert{0pt}{}n k \\ (-1)^k\frac{n!}{k!}\mathcal P^k_n\left(1, \frac1 2, \frac1 3, \dotsc\right) & = \genfrac\{\}{0pt}{}n k \\ (-1)^k\frac{n!}{k!}\mathcal P^k_n\left(1, 0, 0, \dotsc\right) & = \delta_{n, k}. \end{align*}

The right-hand side in the first formula is the Stirling cycle number, the right-hand side in the second formula is the Lah number, right-hand side in the third formula is Stirling set number and the right-hand side in the fourth formula is Kronecker's delta.

It is obvious that $\mathcal P_{n}^{k}(1, 1, 1, ...) = \binom{n-1}{k-1}$ since the definition of the Lah numbers according to Wikipedia is

$$\genfrac\lvert\rvert{0pt}{}n k = \frac{n!}{k!} \binom{n-1}{k-1}.$$

In the blog I do not find the general formula for $\mathcal P_{n}^{k}(a_{1}, a_{2}, \dotsc)$, so my question is: what is the general formula for $\mathcal P_{n}^{k}(a_{1}, a_{2}, \dotsc)$?

Edit: (some questions I came up with after posting this question)

• How is $P_{n}^{k} (a_1, a_2, ...)$ connected to DeMoivre polynomials $A_{n, k}$?

Answer 1

$P_n$ is given as $$P_n(f) = \sum_{\lambda \,\vdash\, n} (-1)^{\lambda_1} \prod \binom{\lambda_j}{\lambda_{j+1}} f_{j}^{\lambda_j}$$ where

• the sum is over partitions $\lambda = \lambda_1 \ge \lambda_2 \ge \cdots$ of $n$;
• $\lambda_{j+1} = 0$ if $j+1$ is greater than the number of parts in $\lambda$;
• I've adjusted the index from $f_{j+1}$ to $f_j$ because it appears that the increment was a hack to 1-index the variables in a 0-indexed programming language.

Then in the section Prototypic examples the text says

So let's define $P^k_n(f)$ as the $P$ transform of $f$ restricted to the partitions of $n$ with largest part $k$

Therefore $$P^k_n(f) = \sum_{\lambda' \,\vdash\, n-k} (-1)^{k} \binom{k}{\lambda'_1} f_1^k \prod \binom{\lambda'_j}{\lambda'_{j+1}} f_{j+1}^{\lambda_j}$$

A combinatorial interpretation of this can be obtained by defining a stalactite diagram (based on the English convention for Ferrers diagrams) to be a collection of cells where the top row is continuous and each cell in a lower row is directly below another cell. E.g. the stalactite diagrams with 4 cells are

If we label the cells in the top row with $-f_1$ and the cells in row $k > 1$ with $f_k$ then $P_n^k(f)$ is the sum over all stalactite diagrams of $n$ cells in $k$ columns of the product of the cells in the diagram. This leads directly into the alternative formula $$P_n^k(f) = [x^n] \left(-f_1 x - f_1 f_2 x^2 - f_1 f_2 f_3 x^3 - \cdots \right)^k$$ and that answers the follow-up question as to the relationship with De Moivre polynomials: $A_{n,k}(a) = [x^n](a_1x + a_2x^2 + a_3x^3 + \cdots)^k$, so $$P_n^k(f) = A_{n,k}(-f_1, -f_1 f_2, -f_1 f_2 f_3, \ldots)$$