Perhaps you should try some spelling variations:
"introduction frontier region" » "introduction fourier region", "introduction front region", "production frontiers region", "introduction frontal region"
"production frontier functions" » "production wannier functions", "reproduction frontier functions"
"production frontier section" » "production frontiers section", "production frontier second", "production frontal sections", "reproduction frontier section"

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: (((("production frontier section") OR ("production frontier functions"))) OR ("introduction frontier region")).
• Removing quotes may allow a broader search: (((( production frontier section ) or ( production frontier functions ))) or ( introduction frontier region )).
• Adding a wildcard symbol may retrieve word variants: (((("production frontier section") or ("production frontier functions"))) or ("introduction frontier region"))*.