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