虚根

虚根	xū gēn	(math.) imaginary root
虚	xū	emptiness / void / abstract theory or guiding principles / empty or unoccupied / diffident or timid / false / humble or modest / (of health) weak / virtual / in vain
根	gēn	root / basis / classifier for long slender objects, e.g. cigarettes, guitar strings / CL:条 / radical (chemistry)