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 section" » "production frontiers section", "production frontier second", "production frontier functions", "introduction 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 ("reproduction frontier region"))*.
• Removing quotes may allow a broader search: (( production frontier section ) or ( reproduction frontier region ))*.