Perhaps you should try some spelling variations:
communication communicating » communication communication
communicating based » communications based, communication based, communicating bad
based dispersion » wave dispersion, gas dispersion, based discussion
index theorem » index theory
index free » index tree, index from

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• Performing a fuzzy search may retrieve terms with similar spellings: communication communicating based dispersion ((index free) or (index theorem))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: communication communicating based dispersion ((index free) OR (index theorem)).
• Adding a wildcard symbol may retrieve word variants: communication communicating based dispersion ((index free) or (index theorem))*.