Perhaps you should try some spelling variations:
"optimization algorithmss" » "optimisation algorithm", "estimation algorithm", "optimized algorithms", "optimisation algorithms", "estimation algorithms", "optimized algorithmss"
"optimization algorithms" » "optimisation algorithm", "estimation algorithm", "optimized algorithms"
"estimation algorithms" » "estimation algorithm", "optimisation algorithm", "approximation algorithms"

Try widening your search to All Fields.

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