自由电子的运动学由自由狄拉克方程——一个四维系统的偏微微分方程描述。
The kinematics of free electrons is described by the free Dirac equation, a four-dimensional system of partial differential equations.
脚微微分开，伸展，双手和视线往上，看向天花板。在伸展的时候感觉到延伸的感觉并深深呼吸。
Feet slightly apart, reach and look up towards the ceiling with your arms and eyes. Feel the stretch and take in a deep breath as you reach up.
有一种笑是嘴唇抿在一起或者稍微分开，嘴唇一头微微扬起。
With this smile the lips are pressed together or slightly separated, with one side of the mouth slightly raised.
有一种笑是嘴唇抿在一起或者稍微分开，嘴唇一头微微扬起。
With this smile the lips are pressed together or slightly separated, with one side of the mouth slightly raised.
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