def ssig_from_a(a, ncounts):
    """Theoretical power in the Z or PDS search for a sinusoid of amplitude a.

    From Leahy et al. 1983, given a pulse profile
    p = lambda * (1 + a * sin(phase)),
    The theoretical value of Z^2_n is Ncounts / 2 * a^2

    Note that if there are multiple sinusoidal components, one can use
    a = sqrt(sum(a_l))
    (Bachetti+2021b)

    Examples
    --------
    >>> round(ssig_from_a(0.1, 30000), 1)
    150.0
    """
    return ncounts / 2 * a ** 2