VSA Hessian Matrix Description
The chi^2 is given by chi^2=-2 log_like= sum_ij
(O_i-log(B_i+x_i)) M_ij (O_j-log(B_j+x_j)) where B_i are theoretical bandpowers,
x_i are the x-factors and O_i are the observed powers in the offset-log-normal
variables (O_i=log(O_i0+x_i)).