def psd_matern_12(omega, lam, lam_t):
    """Spectral density of Matern-1/2 process.

    Args:
        omega (tensor): Frequency.
        lam (tensor): Decay.
        lam_t (tensor): Scale.

    Returns:
        tensor: Spectral density.
    """
    return 2 * lam_t * lam / (lam ** 2 + omega ** 2)