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