'Returns equations that can be solved numerically
Parameters
inv_method : str
The specific sympy inverse matrix calculation method to use. For a
list of valid methods, see
:meth:`~sympy.matrices.matrices.MatrixBase.inv`'

def rhs(self, inv_method=None, **kwargs):

    if (inv_method is None):
        self._rhs = self.mass_matrix_full.LUsolve(self.forcing_full)
    else:
        self._rhs = (self.mass_matrix_full.inv(inv_method, try_block_diag=True) * self.forcing_full)
    return self._rhs
