There are many, many websites that include a search function that returns a list of papers matching some keyword: Scirate, Shortscience, Brevy, etc.
Do all these sites run their own crawlers and build up their own index? Or is there a shared index somewhere?
For example, if you go to shortscience.org/search, and type in 'computer vision' it comes up with a list of relevant papers. Where is this data coming from?