It is surprising that science does not know the answer, though there have been some inventive and controversial ideas. This book considers many of those ideas and presents a new solution why three is the magic...
Детлаф А. А., Яворский Б. М. Курс физики (в трех томах): Учебное пособие.— 3-е изд., перераб. и доп.—М.: Высш. школа, 1979, т. III: Волновые процессы....