Perhaps you should try some spelling variations:
"necessary and sufficient condition" » "necessary and sufficient conditions"
"necessary and efficient conditions" » "necessary and sufficient conditions", "necessary and efficient conditional", "necessary and efficiency conditions"

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: (("necessary and sufficient condition") OR ("necessary and efficient conditions")).
• Removing quotes may allow a broader search: (( necessary and sufficient condition ) or ( necessary and efficient conditions )).
• Adding a wildcard symbol may retrieve word variants: (("necessary and sufficient condition") or ("necessary and efficient conditions"))*.