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

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 passive estimation algorithms ) OR ( modified df approximation algorithm ))~.
• Adding a wildcard symbol may retrieve word variants: (( modified passive estimation algorithms ) or ( modified df approximation algorithm ))*.