I am teaching a combinatorics class in which I introduced the notion of a "mass formula". My terminology is inspired by the Smith–Minkowski–Siegel mass formula for the total mass of positive-definite quadratic forms of a given size and genus. That famous mass formula is much too fancy of an example for my class. All that I really do is define the concept of the "mass" of a combinatorial object to be $1/|G|$ if $G$ is its automorphism group, and then argue that it can be easier to find the total mass of a collection of objects than to count them straight (using Polya counting theory). For example, the total mass of unlabeled trees of order $n$ is $n^{n-2}/n!$, because there are $n^{n-2}$ labeled trees.

So I have two questions for which a quick answer (i.e. sooner than two weeks) would be most convenient:

- Is "mass formula" a standard name for this concept? Is there a standard name?
- Can someone suggest a free on-line reference, comparable to a Wikipedia page or a little longer? The class textbook doesn't have a discussion.