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\begin{table}  \caption {\textbf{Table 1.} Overview of data collected for a cohort of 4,606 articles submitted to the preprint database arXiv between October 4, 2010 and May 2, 2011.}  \begin{tabular}{lrrr}  & N \textbf{N}  & articles \textbf{articles}  & time period\\ \textbf{time period}\\  arXiv downloads & 2,904,816 & 4,606 & October 4, 2010 to May 9, 2011\\   Twitter mentions & 5,752 & 4,415 & October 4, 2010 to May 9, 2011\\  early citations & 431 & 70 & October 4, 2010 to September 30, 2011\\