In the paper, http://people.duke.edu/~hpgavin/ce281/lm.pdf, equations 1), 2), and 3) refer to the chi-squared error criterion.
Equation 2) is reproduced below.
$$ \chi^2({\mathbf p}) = (\mathbf y- \hat{\mathbf y}({\mathbf p}))^{\mathsf T}{\mathbf W}(\mathbf y- \hat{\mathbf y}({\mathbf p})) $$
What does the superscript T notation represent? Equation 2) appears to be short hand notation for equation 1), which sums over i from 1 to m.
I dusting off my old paper (and code) on the Marquardt algorithm from 1966 as well as learning the updates, like Levenberg-Marquardt, that have been made since.
I am not and never was a math major.