  This paper presents a thorough study of negation in SPARQL. The types of
negation supported in SPARQL are identified and their main features discussed.
Then, we study the expressive power of the corresponding negation operators. At
this point, we identify a core SPARQL algebra which could be used instead of
the W3C SPARQL algebra. Finally, we analyze the negation operators in terms of
their compliance with elementary axioms of set theory.
