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