Eqn (8) from Greenhall2004

def greenhall_sx(t, F, alpha):
    """ Eqn (8) from Greenhall2004
    """
    if F == float('inf'):
        return greenhall_sw(t, alpha+2)
    a = 2*greenhall_sw(t, alpha)
    b = greenhall_sw(t-1.0/float(F), alpha)
    c = greenhall_sw(t+1.0/float(F), alpha)

    return pow(F, 2)*(a-b-c)