Perhaps you should try some spelling variations:
"structural foundations" » "structural conditions", "structural fluctuations", "cultural foundations"
"structural equations" » "structural equation", "structural evolution"
"structures functions" » "structure functions", "structure function", "structure functional"

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: (((("structural foundations") OR ("structural equations"))) OR ("structures functions")).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"structural foundations\") OR (\"structural equations\"))) or (\"structures functions\"))".
• Removing quotes may allow a broader search: (((( structural foundations ) OR ( structural equations ))) or ( structures functions )).
• Adding a wildcard symbol may retrieve word variants: (((("structural foundations") OR ("structural equations"))) or ("structures functions"))*.