Perhaps you should try some spelling variations:
"_ optimization algorithm" » "_ optimization algorithms", "_ optimisation algorithm", "_ estimation algorithm"
"time approximation algorithms" » "one approximation algorithms", "_ approximation algorithms"
"_ optimize algorithm" » "_ optimized algorithm", "_ optimized algorithms", "_ optimizer algorithms", "_ optimise algorithm"

Try widening your search to All Fields.

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• Performing a fuzzy search may retrieve terms with similar spellings: (("time approximation algorithms") or ((("_ optimize algorithm") or ("_ optimization algorithm"))))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("time approximation algorithms") OR ((("_ optimize algorithm") OR ("_ optimization algorithm")))).
• Removing quotes may allow a broader search: (( time approximation algorithms ) or ((( _ optimize algorithm ) or ( _ optimization algorithm )))).
• Adding a wildcard symbol may retrieve word variants: (("time approximation algorithms") or ((("_ optimize algorithm") or ("_ optimization algorithm"))))*.