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

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 ((("production frontier region") OR ("reproduction frontier region")))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"production frontier section\") or (\"production frontier functions\"))) or (((\"production frontier region\") OR (\"reproduction frontier region\"))))".
• Removing quotes may allow a broader search: (((( production frontier section ) or ( production frontier functions ))) or ((( production frontier region ) OR ( reproduction frontier region )))).
• Adding a wildcard symbol may retrieve word variants: (((("production frontier section") or ("production frontier functions"))) or ((("production frontier region") OR ("reproduction frontier region"))))*.