19.1.5 B88:

A. D. Becke, Phys. Rev. A. 38, 3098 (1988)

$$K= -\sum_s \rho_{s}^{4/3}\left (c+{\frac {\beta\,\chi_{s} ^{2}}{1+6\,\beta\,\chi_{s}{\,\text{arcsinh}}(\chi_{s})}}\right ) ,$$ (27)

where

$$c=\frac{3}{2}\left (\frac{3}{4\pi}\right )^{1/3}$$ (28)

and

$$\beta= 0.0042 .$$ (29)