This book is a course of lectures given for senior students at the Moscow Institute of Physics and Technology. For those who have graduated in the USSR this information should beisufficient to give an idea of the level and the manner in which the subject matter is presented. On the other hand, readers outside of this country may never have heard about this well-known Sgviet institution, and so we would like to say few words about it now. The Moscow Institute of Physics and Technology (MFTI or Fiztekh) was founded in 1947 as the result of a special directive of Stalin in order to supply the...