The arXiv (pronounced "archive", as if the "X" were the Greek Letter Chi χ) is an archive for electronic preprints of scientific papers in the fields of mathematics, physics, astronomy, computer science, quantitative biology, statistics, and quantitative finance which can be accessed online. In many fields of mathematics and physics, almost all scientific papers are self-archived on the arXiv. On 3 October 2008, arXiv.org passed the half-million article milestone, with roughly five thousand new e-prints added every month. The preprint archive turned 20 years old on 14 August 2011
The Digital Library for Physics and Astronomy is operated by the Smithsonian Astrophysical Observatory under a NASA grant. SAO/NASA Astrophysics Data System (ADS) is a Digital Library portal for researchers in Astronomy and Physics.. The ADS maintains three bibliographic databases containing more than 9.3 million records: Astronomy and Astrophysics, Physics, and arXiv e-prints.