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Electronic Journal of Combinatorics
Article . 1995 . Peer-reviewed
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An Extension of the Exponential Formula in Enumerative Combinatorics

An extension of the exponential formula in enumerative combinatorics
Authors: Gilbert Labelle; Pierre Leroux;

An Extension of the Exponential Formula in Enumerative Combinatorics

Abstract

Let $\alpha$ be a formal variable and $F_w$ be a weighted species of structures (class of structures closed under weight-preserving isomorphisms) of the form ${F}_{w} = E({F}_{w}^{c})$, where $E$ and $F_w^c$ respectively denote the species of sets and of connected $F_w$-structures. Multiplying by $\alpha$ the weight of each $F_w^c$-structure yields the species ${F}_{{w}^{( \alpha )}} = E({F}_{ \alpha w}^{c})$. We introduce a "universal" virtual weighted species, $\Lambda ^{(\alpha)}$, such that $F_{w^{(\alpha)}} = \Lambda^{( \alpha)}\, \circ \, F_w^+$, where $F_w^+$ denotes the species of non-empty $F_w$-structures. Using general properties of $\Lambda^{( \alpha)}$ , we compute the various enumerative power series $G(x)$, $\widetilde{G}(x)$, $\overline{G}(x)$, $G(x;q)$, $G\langle{x;q}\rangle$, ${Z}_{G}(x_1,x_2,x_3,\ldots)$, ${\Gamma }_{G}(x_1,x_2,x_3,\ldots)$, for $G = F_{w^{(\alpha)}}$, in terms of $F_w$. Special instances of our formulas include the exponential formula, ${F}_{{w}^{(\alpha )}}(x)=\exp(\alpha F_{w}(x))=({F}_{w}(x){)}^{\alpha }$, cyclotomic identities, and their $q$-analogues. The virtual weighted species, $\Lambda ^{(\alpha)}$, is, in fact, a new combinatorial lifting of the function ${(1+x)}^{\alpha }$.

Related Organizations
Keywords

enumerative power series, exponential formula, cyclotomic identities, Exact enumeration problems, generating functions, weighted species, structures

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citations
This is an alternative to the "Influence" indicator, which also reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Citations provided by BIP!
popularity
This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network.
BIP!Popularity provided by BIP!
influence
This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Influence provided by BIP!
impulse
This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network.
BIP!Impulse provided by BIP!
7
Average
Average
Average
gold