Searching books and papers with equations
You can perform $\LaTeX$ search - that is, write formula in LaTeX in an appropriate search engine:
- http://www.latexsearch.com/
However, as one can type the same expression in different ways and with different symbols, I never used it it practice. (Anyone did?)
There was a similar question at Mathoverflow. I think it contains some useful references and discussion so it's definitely worth checking out.
Usually I would post this as a comment under the question but probably nobody would notice it anymore. And seeing that the other answer is also quite short I hope this is fine.