This is pretty neat. It's annoying though that this doesn't specify where it searches. When this was discussed earlier on Reddit[0], the person who made it said that it (as of 2 months ago) searches Wikipedia, Wolfram MathWorld, NIST DLMF, MathOverflow and PlanetMath, but that should probably be specified somewhere on the site. And as lots of people have already said, to be really useful this should search arXiv.

But this is nitpicking. This is still the first time I've seen someone make something like this, so, again, really neat!

[0]https://www.reddit.com/r/math/comments/2po75u/searchonmath_a...

Edit: See rhambasan's comment which has a link to an arXiv math searcher; I haven't tested it to see how it compares.