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