Perhaps you should try some spelling variations:
approximation algorithm » approximation algorithms
estimation algorithms » estimation algorithm, optimization algorithms, optimisation algorithm
passive estimation » pose estimation
bi1 approximation » time approximation, _ approximation, one approximation
modified passive » modified adaptive
modified bi1 » modified _

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• The words AND, OR and NOT may confuse the search; try adding quotes: "(( modified passive estimation algorithms ) OR ( modified bi1 approximation algorithm ))~".
• Adding a wildcard symbol may retrieve word variants: (( modified passive estimation algorithms ) OR ( modified bi1 approximation algorithm ))*.