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

Try widening your search to All Fields.

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• If you are trying to use Boolean operators, they must be ALL CAPS: (("introduction frontier section") OR ("production frontier region")).
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