Perhaps you should try some spelling variations:
observations theory » observations thereon, observations towards
operations theory » operators theory, operator theory, options theory
operator » operation
operators » operations

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: (( (operator OR operators) theory ) OR ((( ((operations theory) OR (relations theory)) ) OR ( observations theory )))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "(( (operator OR operators) theory ) or ((( ((operations theory) or (relations theory)) ) or ( observations theory ))))".
• Adding a wildcard symbol may retrieve word variants: (( (operator OR operators) theory ) or ((( ((operations theory) or (relations theory)) ) or ( observations theory ))))*.