SINGULAR EFFECTS IN PHONON FOCUSING ; FORMATION OF PHONON FOCUSINGCAUSTICS J.C. Hensel, R.C. Dynes, F.C. Unterwald et A.L. Simons J. Phys. Colloques, 42 C6 (1981) C6-212-C6-214 DOI: 10.1051/jphyscol:1981661