Perhaps you should try some spelling variations:
"difference and function relationship" » "difference and function relationships"
"difference and function equations" » "difference and functional equations", "difference and functional equation", "difference and function relations"

Try widening your search to All Fields.

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: (("difference and function equations") OR ("difference and function relationship")).
• Removing quotes may allow a broader search: (( difference and function equations ) or ( difference and function relationship )).
• Adding a wildcard symbol may retrieve word variants: (("difference and function equations") or ("difference and function relationship"))*.