Perhaps you should try some spelling variations:
"operations theory" » "operators theory", "observations theory", "relations theory"
"operation theory" » "iteration theory", "generation theory", "operation theatre"
"operator theory" » "operators theory"
"aberration test" » "aberration text"

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 ((((("aberration theory") OR ("aberration test"))) OR ("operations theory")))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"operator theory\") OR (\"operation theory\"))) or (((((\"aberration theory\") or (\"aberration test\"))) or (\"operations theory\"))))".
• Removing quotes may allow a broader search: (((( operator theory ) OR ( operation theory ))) or ((((( aberration theory ) or ( aberration test ))) or ( operations theory )))).
• Adding a wildcard symbol may retrieve word variants: (((("operator theory") OR ("operation theory"))) or ((((("aberration theory") or ("aberration test"))) or ("operations theory"))))*.