Takada has developed a new method [1] to treat nonparabolicity of the conduction bands in order to obtain electronic structures [2,3] as well as optical absorption spectra [4] in the inversion layers formed on the surface of narrow-gap semiconductors such as InSb and Hg1-xCdxTe. Since electrons occupy two or more subbands in the system, there exists a possibility to observe excitations of  acoustic plasmons [5].    Occupation of multiple subbands is also possible in the inversion layers on the suface of Si under the uniaxial stress. The electronic structure, the transport properties, and the cyclotron resonance are considered in the system [6-8].