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

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 functions") OR ("structural equations"))) OR ("structural functional")).
• Removing quotes may allow a broader search: (((( structural functions ) or ( structural equations ))) or ( structural functional )).
• Adding a wildcard symbol may retrieve word variants: (((("structural functions") or ("structural equations"))) or ("structural functional"))*.