Name the author who published the paper on Retrieval approach and has the name Petkova.
Which author published the paper on Retrieval approach and has the name Petkova?
SELECT DISTINCT ?answer WHERE { <https://dblp.org/rec/conf/clef/BallesterosP05> <https://dblp.org/rdf/schema#authoredBy> ?answer }
<https://dblp.org/rec/conf/clef/BallesterosP05>