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 cor- responding negation operators.
At this point, we identify a core SPARQL algebra which could be used instead of
the W3C SPARQL algebra. Fi- nally, we analyze the negation operators in terms
of their compliance with elementary axioms of set theory.