Soft-thresholding (软阈值) operator

S ( z , γ ) S(z,\gamma) S(z,γ) is the soft-thresholding operator with value

S ( z , γ ) = s i g n ( z ) ( ∣ z ∣ − γ ) + = { z − r , i f z > 0 a n d γ < ∣ z ∣ , z + r , i f z < 0 a n d γ < ∣ z ∣ , 0 , i f γ ≥ ∣ z ∣ . S(z,\gamma)={\rm{sign}}(z)(|z|-\gamma)_+ = \left\{ \begin{array}{rcl} z-r, \quad if &z>0 &and& \gamma<|z|, & \\ z+r, \quad if &z<0 &and&\gamma<|z|, & \\ 0, \quad if &\gamma \geq |z|. & \end{array} \right. S(z,γ)=sign(z)(zγ)+=zr,ifz+r,if0,ifz>0z<0γz.andandγ<z,γ<z,

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