In that paper, they consider the following production function:
$Y = F(A_KK,A_LL)$.
K denotes capital, L is labor, and $A_K$ and $A_L$ denote capital-augmenting and labor-augmenting technology, respectively. We assume throughout that F is continuously differentiable, concave, and exhibits constant returns to scale. Let $F_K$ and $F_L$ denote the derivatives of F with respect to capital and labor. We focus on competitive labor markets, which implies that the equilibrium wages is equal to the marginal product of labor:
$W = A_L F_L(A_KK,A_LL)$.
Why? Shouldn't simply be:
$W = F_L(A_KK,A_LL)$.
Without the $A_L$ multiplying the derivative?