def METHOD_NAME(self, inputs, outputs, residuals):
    """
    R = Ax - b.
    Parameters
    ----------
    inputs : Vector
        Unscaled, dimensional input variables read via inputs[key].
    outputs : Vector
        Unscaled, dimensional output variables read via outputs[key].
    residuals : Vector
        Unscaled, dimensional residuals written to via residuals[key].
    """
    if self.options['vec_size'] > 1:
        if self.vec_size_A > 1:
            residuals['x'] = np.einsum('ijk,ik->ij', inputs['A'], outputs['x']) - inputs['b']
        else:
            residuals['x'] = np.einsum('jk,ik->ij', inputs['A'], outputs['x']) - inputs['b']
    else:
        residuals['x'] = inputs['A'].dot(outputs['x']) - inputs['b']