A lot of us (especially professional mathematicians) here, in posting answers and questions, link to one of the four commonly used resources: Wikipedia, the Springer encyclopedia of mathematics, PlanetMath, and Wolfram MathWorld.
What is unthinkable to me just happened: all links to the Springer EOM are, as of this moment, broken. (This is due to their transitioning to a new Wiki-based model, and acquiring a new domain name. You can read my complaint in the link above, but that is not the point of this post.)
I am secretly hoping that Springer would do "the right thing" and insert proper redirects from their old website to the new one (making DOI seems like a good idea now also). But in case that doesn't happen:
Here's a plea to the good samaritans of Math.SE: if you see a broken link to
eom.springer.com, hop on over to
www.encyclopediaofmath.organd find the correct article (all EOM articles have been imported to that Wiki to seed the database), and replace the link.