Perhaps you should try some spelling variations:
observations theory » observations thereon, observations towards
operations theory » relations theory, options theory, vibrations 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 theory ) OR ((( ((operations theory) OR (operators theory)) ) OR ( observations theory )))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "(( operator theory ) or ((( ((operations theory) OR (operators theory)) ) or ( observations theory ))))".
• Adding a wildcard symbol may retrieve word variants: (( operator theory ) or ((( ((operations theory) OR (operators theory)) ) or ( observations theory ))))*.