Perhaps you should try some spelling variations:
"modified allocation approximation algorithms" » "modified allocation optimization algorithms"
"modified staffing estimation algorithms" » "modified staffing estimation algorithm", "modified staffing optimization algorithms", "modified staffing optimisation algorithm", "modified caging estimation algorithms"

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: (("modified allocation approximation algorithms") OR ("modified staffing estimation algorithms"))~.
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