空符号是相对于有声有色有定形的实符号而言。
The empty sign is opposite to the vivid, real, fix-formed sign.
函数是一系列实函数，其函数值的符号完全取决于其各个变量的符号。
An R-function is a real-valued function whose sign is completely determined by the signs of its arguments.
若在适当端口施加正弦电压源，则网络的稳定性取决于进入端口基波电流的实功与虚功成份的符号值。
When exciting Sin voltage source is applied to proper port, the stability of network depends on the sign of active and reactive components for fundamental-wave current entering port of networks.
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