def ZSBB_mean_coordination_number(b0,lam,N,b1):
    """
    Compute the mean coordination number of the ZSBB model.
    """

    s = 0.0
    for n in range(1,N):
        s += (n+1.0) / ((n+1.0)*b1 - 1.0) * ((1.0-lam)/lam)**(n-1)

    pi_10 = ( 0.5 / ( b0 - (2*lam - 1.0)/(3*lam - 1.0)) + 0.5/lam * s )**(-1.0)

    return pi_10 / (2.0*lam) * s