Perhaps you should try some spelling variations:
"modified df approximation algorithm" » "modified df approximation algorithms", "modified _ approximation algorithm", "modified one approximation algorithm", "modified time approximation algorithm", "modified adam approximation algorithm", "modified bi1 approximation algorithm"
"modified objective optimisation algorithm" » "modified objective optimization algorithm", "modified multiobjective optimization algorithm", "modified objective optimization algorithms", "modified objective estimation algorithm", "modified objective 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 df approximation algorithm") OR ("modified objective optimisation algorithm"))~.
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