A species of structures is a rule which

- 1. Produces, for each finite set , a finite set ,
- 2. Produces, for each bijection , a function

The functions should further satisfy the following functorial properties:

- 1. For all bijections and ,

- 2. For the Identity Map
,

An element is called an -structure on (or a structure of species on ). The function is called the transport of -structures along .

**References**

Bergeron, F.; Labelle, G.; and Leroux, P. *Combinatorial Species and Tree-Like Structures.*
Cambridge, England: Cambridge University Press, p. 5, 1998.

© 1996-9

1999-05-26