Perhaps you should try some spelling variations:
observations towards » observations taken, observations theory
operation theory » operations theory, generation theory, aberration theory
iteration theory » integration theory, interaction theory, vibration theory
operator » operators

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 ((( ((operation theory) OR (iteration theory)) ) OR ( observations towards )))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "(( operator theory ) or ((( ((operation theory) OR (iteration theory)) ) or ( observations towards ))))".
• Adding a wildcard symbol may retrieve word variants: (( operator theory ) or ((( ((operation theory) OR (iteration theory)) ) or ( observations towards ))))*.