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