In this 2005 Notices article, Jerold Grossman tracks the proportion of papers in Math Reviews with 1, 2, 3, and >3 authors over time. His data set ends in 1999. I seem to recall reading that in 200k, for some value of k, the number of collaborative papers in math exceeded the number of single-authored papers for the first time. But I can't find any record of this. Does anyone know what k is, what a reliable source for this kind of data is, and what the proportion of collaborative papers is now?