Perhaps you should try some spelling variations:
"observations theory" » "observations thereon", "observations towards"
"operation theory" » "operations theory", "operator theory", "generation 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: (("operation theory") OR ((("iteration theory") OR ("observations theory")))).
• Removing quotes may allow a broader search: (( operation theory ) or ((( iteration theory ) or ( observations theory )))).
• Adding a wildcard symbol may retrieve word variants: (("operation theory") or ((("iteration theory") or ("observations theory"))))*.