An algebraic structure whose elements consist of selected Homeomorphisms between open subsets of a Space, with the composition of two transformations defined on the largest possible domain. The ``germs'' of the elements of a pseudogroup form a Groupoid (Weinstein 1996).

**References**

Weinstein, A. ``Groupoids: Unifying Internal and External Symmetry.'' *Not. Amer. Math. Soc.* **43**, 744-752, 1996.

© 1996-9

1999-05-26