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

You may be able to get more results by adjusting your search query.

• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"production frontier section\") OR (\"production frontier region\"))*".
• Removing quotes may allow a broader search: (( production frontier section ) OR ( production frontier region ))*.