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In symbolic method, one often considers two operators on ordinary generating functions, namely

$$ \operatorname{PSET}F(x) = \exp\left(F(x)-\frac{F(x^2)}{2}+\frac{F(x^3)}{3}-\dots\right), $$

and

$$ \operatorname{MSET}F(x) = \exp\left(F(x)+\frac{F(x^2)}{2}+\frac{F(x^3)}{3}+\dots\right). $$

These operators allowed to enumerate sets (for $\operatorname{PSET}$) or multisets (for $\operatorname{MSET}$) constructed of unlabeled $F$-structures.

One may note that $\operatorname{MSET} F(x)$ is, essentially, unlabeled generating functions for the species $E \circ F$, where $E$ is the species of sets and $\circ$ denotes the composition of species. Correspondingly, the relevant exponential generating function would be $e^{F(x)}$.

Is there any meaningful way to interpret $\operatorname{PSET}$ in the terms of labeled species as well? Ideally, to get some formula for the exponential generating function of the labeled version of $\operatorname{PSET}$.

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1 Answer 1

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See Gilbert Labelle, On asymmetric structures, Discrete Math. 99 (1992), 141–164.

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  • $\begingroup$ Oh, nice! So, PSET may be interpreted as flat part of $E \circ F$, if taken over flat parts of $F$. Although this is quite close to what I was asking for, I'm not sure it covers everything? I mean, PSET seems to be well-defined for any unlabeled species, not necessarily flat ones... $\endgroup$ Commented Jul 20, 2022 at 9:54
  • $\begingroup$ By the way, thanks for this presentation, it contains a lot of invaluable examples and intuition, which I used to introduce species to my peers! $\endgroup$ Commented Jul 20, 2022 at 9:56

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