The Springer LaTeX search lets you search through over   LaTeX code snippets to find the equation you need.
Sample Searches
Sample Result
Acta Mathematica Hungarica (2002) 96:187-220, August 01, 2002
P\left( {E_n^{\left( d \right)} ,{\text{i}}{\text{.o}}{\text{.}}} \right) = 0\quad {\text{or}}\quad {\text{1}}
LaTeX Search ... allows researchers to search for LaTeX formatted equations in all of Springer's journals. That's something you can't do with Google, or any other search engine.The ability to connect obscure mathematical discoveries from disparate fields of science could soon be facilitating new avenues of research, perhaps even new methodolgies.
LaTeX Search does what it promises, using similarity algorithms "to normalize and compare LaTeX strings so that, if similar equations are written slightly differently, the outputs are normalized and matched, granting you the broadest possible results set".
– Dario Taraborelli, http://www.academicproductivity.com