Lambert's W functions, @math{W(x)}, are defined to be solutions
of the equation @math{W(x) \exp(W(x)) = x}. This function has
multiple branches for @math{x < 0}; however, it has only
two real-valued branches. We define @math{W_0(x)} to be the
principal branch, where @math{W > -1} for @math{x < 0}, and
@math{W_{-1}(x)} to be the other real branch, where
@math{W < -1} for @math{x < 0}. The Lambert functions are
declared in the header file ``gsl_sf_lambert.h'`.

__Function:__double**gsl_sf_lambert_W0***(double*`x`)__Function:__int**gsl_sf_lambert_W0_e***(double*`x`, gsl_sf_result *`result`)- These compute the principal branch of the Lambert W function, @math{W_0(x)}.

__Function:__double**gsl_sf_lambert_Wm1***(double*`x`)__Function:__int**gsl_sf_lambert_Wm1_e***(double*`x`, gsl_sf_result *`result`)- These compute the secondary real-valued branch of the Lambert W function, @math{W_{-1}(x)}.

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