Perhaps you should try some spelling variations:
observations theory » observations thereon, observations towards
operators theory » operator theory
iteration theory » integration theory, interaction theory, vibration theory

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: (( operators theory ) OR ((( iteration theory ) OR ( observations theory )))).
• Adding a wildcard symbol may retrieve word variants: (( operators theory ) or ((( iteration theory ) or ( observations theory ))))*.