def loss_l2(self, l2=0):
        """L2 loss centered around mu_init, scaled optionally per-source.

        In other words, diagonal Tikhonov regularization,
            ||D(\mu-\mu_{init})||_2^2
        where D is diagonal.

        Args:
            - l2: A float or np.array representing the per-source regularization
                strengths to use
        """
        if isinstance(l2, (int, float)):
            D = l2 * torch.eye(self.d)
        else:
            D = torch.diag(torch.from_numpy(l2))

        # Note that mu is a matrix and this is the *Frobenius norm*
        return torch.norm(D @ (self.mu - self.mu_init)) ** 2