Best citations database Which online service provides the most complete list of citations to given mathematical paper? I mean the citations both in published papers and in preprints. I guess scholar.google is the best, maybe there is something more powerfull? 
 A: In my experience google gives you much more than others, but it is not "clean"
There are also
Those who have subscription (me not) use MathSciNet 
http://www.ams.org/mathscinet/
Or european analogue
http://www.zentralblatt-math.org/zmath/
Also there is Russian 
http://www.mathnet.ru/
But it is not so developed. But it is free.
Free:
http://academic.research.microsoft.com/
Free:
http://www.citebase.org/
Publishing houses like Springer Elseveir provide citation services, but it is mainly within their own journals...
A: Apart from the ones Alexander Chervov has mentioned, there are:


*

*The SAO/NASA Astrophysics Data System

*(Deprecated as of 2018 - old site now hosts porn) Eprintweb
A: Thought this might be interesting.
I became curious and googled for "mathematics citation database".
Could not find anything not mentioned here already, except for the following.
At http://mathematics.library.cornell.edu/find/Mathematics-Databases they have
Jahrbuch-Project Electronic Research Archive for Mathematics. A resource for searching mathematics literature from from 1868 to 1943.
Going there, it is based (I believe) on the Zentralblatt interface, but seems to be particularly sharpened towards old research. They say in particular

17 772 links to facsimiles

Might be useful I think.
