Product of the derivative of our system matrix with respect to the conductivity model and a vector

$\frac{\mathbf{A}(\mathbf{m}) \mathbf{v}}{d \mathbf{m}_{\sigma}} = i \omega \frac{d \mathbf{M^e_{\sigma}}(\mathbf{u})\mathbf{v} }{d\mathbf{m}}$
Parameters
• freq (float) – frequency

• u (numpy.ndarray) – solution vector (nE,)

• v (numpy.ndarray) – vector to take prodct with (nP,) or (nD,) for adjoint