Perhaps you should try some spelling variations:
"modified coverage approximation algorithms" » "modified average approximation algorithms"
"certified network optimization algorithm" » "certified network optimization algorithms", "certified network optimisation algorithm", "certified network estimation algorithm", "verified network optimization algorithm"

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 coverage approximation algorithms") OR ("certified network optimization algorithm"))~.
• Removing quotes may allow a broader search: (( modified coverage approximation algorithms ) or ( certified network optimization algorithm ))~.
• Adding a wildcard symbol may retrieve word variants: (("modified coverage approximation algorithms") or ("certified network optimization algorithm"))*.