Perhaps you should try some spelling variations:
"modified mathematical optimization algorithm" » "modified mathematical optimization algorithms", "modified mathematical optimisation algorithm", "modified mathematical estimation algorithm"
"modified notion approximation algorithms" » "modified network approximation algorithms", "modified time approximation algorithms", "modified one approximation algorithms"

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• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"modified mathematical optimization algorithm\") OR (\"modified notion approximation algorithms\"))~".
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