Does citation refer to paper or authors? I imagine this question is asked before, but I could not find the answer. 

This is the example sentence that got me wondering:


  Gneiting and Ranjan (2013) generalize/generalizes these findings and state/states that ...


Should I plural verbs (generalize, state), as if I am referring to the authors, or should I use singular verbs (generalizes,states) as if I am referring to the paper itself?
I guess there is no definitive answer, but I would use "state". From what I learned from my supervisor, the citation should not interfere with the original sentence. And the sentence is "Gneiting and Ranjan generalize these findings and state that..."
(My supervisor also disapproved of it when someone used "[XY] stated that ... ".) 

Also, I can't remember ever seeing it the other way.

Just make sure that you have the same form for "generalizes" as well ;)