Perhaps you should try some spelling variations:
"observations theory" » "observations towards"
"aberration theory" » "aberration test"
"operator theory" » "operation theory"
"operators theory" » "operations theory"
"observations thereon" » "observations thereof", "observations therein"

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