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