Perhaps you should try some spelling variations:
"operations theory" » "operators theory", "observations theory", "relations theory"
"generation theory" » "generation theorem", "generative theory", "iteration theory"
"operation theory" » "iteration theory", "aberration theory", "operation theatre"
"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 ("operations theory"))) OR ("generation theory")))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"operator theory\") or (((((\"operation theory\") OR (\"operations theory\"))) or (\"generation theory\"))))".
• Removing quotes may allow a broader search: (( operator theory ) or ((((( operation theory ) OR ( operations theory ))) or ( generation theory )))).
• Adding a wildcard symbol may retrieve word variants: (("operator theory") or ((((("operation theory") OR ("operations theory"))) or ("generation theory"))))*.