Perhaps you should try some spelling variations:
"parabolic differential equation" » "partial differential equation", "parabolic differential equations", "parabolic difference equations"
"metabolic differences equations" » "metabolic disturbances equations", "ethnic differences equations", "metabolism references equations", "metabolic difference equations", "metabolic difference equation", "metabolic differential equations"

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• If you are trying to use Boolean operators, they must be ALL CAPS: (("parabolic differential equation") OR ("metabolic differences equations")).
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