Perhaps you should try some spelling variations:
"generative theory" » "generalized theory", "integrative theory", "alternative theory"
"aberration theory" » "aberration test"
"operation theory" » "operations theory", "generation theory", "operation theatre"
"iteration theory" » "integration theory", "interaction theory", "vibration theory"
"operator theory" » "operators 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 ((((("operation theory") OR ("aberration theory"))) OR ((("iteration theory") OR ("generative theory")))))).
• Removing quotes may allow a broader search: (( operator theory ) or ((((( operation theory ) or ( aberration theory ))) or ((( iteration theory ) or ( generative theory )))))).
• Adding a wildcard symbol may retrieve word variants: (("operator theory") or ((((("operation theory") or ("aberration theory"))) or ((("iteration theory") or ("generative theory"))))))*.