Negative resistance circuit
Negative resistance happens when the circuit sources current. This doesn't happen with passive devices like resistors or capacitors. Amplifiers can sink and source current, in this circuit the amplifier will source current as the current through R1 increases. If the circuit is solved it will look like a resistor with negative resistance.

R = -I/V

This is how the opamp behaves. 


Source: https://en.wikipedia.org/wiki/Negative_impedance_converter