def fn_C_crystal_TE_c(l,a,b,beta_11):
    """Clamped capacitance for thickness expander mode, as a function of:
    -- crystal thickness l = l_1
    -- transverse aspect ratios a = l_2 / l_1 and b = l_3 / l_1
    -- impermittivity tensor component beta_11.
    """
    return a * b * l / beta_11