Tamanoi equation for orbifold Euler characteristics: revisited

TL;DR

This paper revisits Tamanoi's Macdonald type equation for orbifold Euler characteristics, explaining its derivation from finite group actions and exploring its limitations for generalized invariants.

Contribution

It clarifies the derivation of Tamanoi's equation from finite group actions and investigates its validity for generalized orbifold Euler characteristics.

Findings

Tamanoi's equation follows from finite group actions on a point.

The equation does not hold for generalized orbifold Euler characteristics in general.

The paper extends the analysis to finitely generated groups.

Abstract

Tamanoi equation is a Macdonald type equation for the orbifold Euler characteristic and for its analogues of higher orders. It claims that the generating series of the orbifold Euler characteristics of a fixed order of analogues of the symmetric powers for a space with a finite group action can be represented as a certain unified (explicitly written) power series in the exponent equal to the orbifold Euler characteristic of the same order of the space itself. In the paper, in particular, we explain how the Tamanoi equation follows from its verification for actions of (finite) groups on the one-point space. Statements used for that are generalized to analogues of the orbifold Euler characteristic corresponding to finitely generated groups. It is shown that, for these generalizatins, the analogue of the Tamanoi equation does not hold in general.