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