Perhaps you should try some spelling variations:
5.5differential equations » b.differential equations, differential equations, ofdifferential equations
22differential equations » differential equations, b.differential equations, ofdifferential equations
2c partial » 22 partial, 23 partial, 24 partial
c partial » 8 partial, 9 partial

Try widening your search to All Fields.

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• If you are trying to use Boolean operators, they must be ALL CAPS: %((22differential equations) OR (5.5differential equations))%((2c partial) OR (c partial))%22.
• The words AND, OR and NOT may confuse the search; try adding quotes: "%((22differential equations) or (5.5differential equations))%((2c partial) OR (c partial))%22".
• Adding a wildcard symbol may retrieve word variants: %((22differential equations) or (5.5differential equations))%((2c partial) OR (c partial))%22*.