I have a question about the Newton optimization with Tikhonov regularization used in your paper

First, thanks for your excellent job!
Could you tell me how to infer the formula (21) in the paper of **RBGT**. I remember that **g** equals (J(x)^T)*f(x). However, the **g^T** in the formula (24) equals J(x). (J(x) is the jacobian matrix) The value of f(x) seems to be not multiplied into the formula. I know that f(x) is the probabilistic function, which is shown in the formula (16).
Or f(θ) equals 1 when θ=0？