Perhaps you should try some spelling variations:
observations theory » observations thereon, observations towards
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 theory )))).
• Adding a wildcard symbol may retrieve word variants: (( operator theory ) or ((( ((operation theory) or (iteration theory)) ) or ( observations theory ))))*.