  There is considerable interest in the citation count for an author's
publications. This has led to many proposals for citation indices for
characterizing citation distributions. However, there is so far no tractable
model to facilitate the analysis of these distributions and the design of these
indices. This paper presents a simple equation for such design and analysis.
The equation has three parameters that are calibrated by three geometrical
characteristics of a citation distribution. Its simple form makes it tractable.
To demonstrate, the equation is used to derive closed-form expressions for
various citation indices, analyze the effect of time and identify individual
contribution to the Hirsch index for a group.
