1. Google Advertising. /ads (last accessed November 2005).
2. Search engine robots. /searchengines/webbots.html (last accessed October 2005).
3. Search Engine Watch: Tips about Internet Search Engines and Search Engine Submission. / (last accessed October 2005).
4. The Web Robots Pages. / (last accessed October 2005).
5. Google and Yahoo! accused of click fraud collusion. ElectricNews.Net, April 5, 2005. .