Eqn (9) from Greenhall2004

def greenhall_sz(t, F, alpha, d):
    """ Eqn (9) from Greenhall2004 """
    if d == 1:
        a = 2*greenhall_sx(t, F, alpha)
        b = greenhall_sx(t-1.0, F, alpha)
        c = greenhall_sx(t+1.0, F, alpha)
        return a-b-c
    elif d == 2:
        a = 6*greenhall_sx(t, F, alpha)
        b = 4*greenhall_sx(t-1.0, F, alpha)
        c = 4*greenhall_sx(t+1.0, F, alpha)
        dd = greenhall_sx(t-2.0, F, alpha)
        e = greenhall_sx(t+2.0, F, alpha)
        return a-b-c+dd+e
    elif d == 3:
        a = 20.0*greenhall_sx(t, F, alpha)
        b = 15.0*greenhall_sx(t-1.0, F, alpha)
        c = 15.0*greenhall_sx(t+1.0, F, alpha)
        dd = 6.0*greenhall_sx(t-2.0, F, alpha)
        e = 6.0*greenhall_sx(t+2.0, F, alpha)
        f = greenhall_sx(t-3.0, F, alpha)
        g = greenhall_sx(t+3.0, F, alpha)
        return a-b-c+dd+e-f-g

    assert(0)